D aniel Day-Lewis is brewing up in the homely basement kitchen of his country house in rural Ireland. 'Actors,' he muses as he pours the boiling water on to the tea bag in the mug, 'should never give interviews. Once you know what colour socks they wear, you'll remember it next time you see them performing, and it will get in the way. It is not in anyone's interest.'

As the preamble to an interview it is not promising, but Day-Lewis has been in the film business for more than 20 years since he first made headlines as the gay hoodlum in My Beautiful Laundrette in 1985, and so knows that, as part of what he labels 'the fatuous science of marketing', talking about himself goes with the territory.

The driver who fetched me from Dublin airport to take me to the actor's home in the heart of the Wicklow mountains is one Day-Lewis himself uses regularly. As we pass through one village en route he points out the pub. 'That's where Daniel sometimes likes to stop for a drink.' With any other film star you can imagine it being something of a visitation, complete with cameras and a blue plaque afterwards. But the man behind the wheel makes it clear that with Day-Lewis it is just what he says it is - dropping in for a pint with an old friend. He has no truck with the trappings of celebrity.

The point is emphasised when we arrive, through woods and via a gently rising track, at the house he shares with his wife, the writer and director Rebecca Miller, daughter of Arthur Miller (they met in 1996 when he was filming an adaptation of The Crucible) and their two young sons, Ronan and Cashel. From the outside its modest Georgian good looks - door in the middle and a window on either side, like the Play School house - suggest the perfect residence for Cecil Vyse, the pompous suitor that Day-Lewis played in the 1986 Merchant Ivory adaptation of EM Forster's A Room with a View. But inside, it is just like any other large, modern family home - children's toys lying about, holiday pictures, coats, caps, scarves, boots, cats, all the paraphernalia of a 'normal' existence.

Day-Lewis once remarked of himself that he had made a 'lifelong study in evasion', and the process of publicising a film is clearly not one he enjoys. It makes him sound grudging about the invasion of his privacy, and generally grumpy - which today couldn't be further from the truth. There is rather an old-fashioned courtliness about him as he leads me, clutching my tea and biscuits, up the curving stone staircase to his green-painted, book-littered ground-floor study. But the remark has lived on - it is quoted on various websites dedicated to the enigma of Day-Lewis - because it chimes readily with the popular image of him, namely that, for all his extraordinary talent, he is a tortured genius, living the life of a recluse, reluctantly breaking cover once every few years to inhabit body and soul hugely demanding screen roles such as Gerry Conlon, victim of a spectacular miscarriage of justice, in In the Name of the Father (1993), or Christy Brown, the Irish writer and painter, born with cerebral palsy, in My Left Foot (1989), or Bill 'The Butcher' in Martin Scorsese's Gangs of New York (2002).

To which impressive list, next month, will be added Daniel Plainview, pioneering oilman, in Paul Thomas Anderson's There Will Be Blood . It is a a visionary, rivetingly strange film centring on Day-Lewis's towering, intensely physical portrait of a haunted and haunting man who strikes it rich in California in the early 1900s. Day-Lewis, who does all his own stunts, is in shot for virtually every one of the 158 minutes running length. It has already won him critics' awards, a nomination for a Golden Globe and talk of a second Oscar to go with the one for My Left Foot. All three New York Times movie critics chose him last week as their top candidate to win best actor this year.

People do cling to the 'mad bastard' stereotype of him, Day-Lewis acknowledges, as we settle in front of the fire. He laughs, a self-deprecating, chuckle at the irony of it all. 'How can you be a recluse,' he asks, 'in a house full of children, even if you had the inclination to be, which I don't? '

Before we go any further I should declare an interest. I've been in this house before, when researching a biography of Day-Lewis's father, the Anglo-Irish poet laureate Cecil Day-Lewis. We talked at length about the family history as Daniel went about his domestic routine, even offering at one stage to take me for a spin on one of his beloved motor bikes. As a father himself - he also has a 12-year-old son, Gabriel-Kane, with the French actress Isabelle Adjani - Daniel looked back on his own childhood, growing up in Greenwich, south London, in the shadow of his feted father. When Day-Lewis was named laureate at the start of 1968, the family home was besieged and the papers were full of pictures of the poet, his wife - the actress Jill Balcon - and their two smiling young children, Daniel and his sister, Tamasin, now a documentary-maker and food writer. The London Evening Standard headlined it: 'A Swinging laureate for the Swinging Sixties.'

Though Day-Lewis died of cancer just four years later, when Daniel was only 15, their two stories are inevitably entwined. Day-Lewis senior, for example, wrote one of his better poems, 'The Newborn', to celebrate the arrival of his son in 1957, including the prophetic lines:

This morsel of man I've held -
What potency it has,
Though strengthless still and naked as
A nut unshelled!

Many assume the best-remembered Day-Lewis poem, 'Walking Away' - about a parent waving a child off at the school gate and musing on 'How selfhood begins with a walking away/ And love is proved in the letting ago' - is about Daniel. It was, however, penned much earlier, about Sean, Day-Lewis's son from his first marriage.

One of the reasons Daniel Day-Lewis now lives in Ireland, he says, is the memory of happy childhood summer holidays in County Mayo when his father would return to the land of his birth. 'I have many images that come from that time,' he recalls. 'The light, the smell, the utter delight with which we would fly out of the car as soon as we arrived and dive into the nearest bit of the Atlantic. The power of them remains undiminished. Life in England was, by comparison, a little colourless. Ireland was a place for the renewal of hope and I still see it like that. It was the place we were all together as a family. And it was like a secret garden. Making a conscious decision to live in a place means you are going to take the mystery out of it to some extent, but you can never entirely do that here. It's one of the great qualities of this place. When people say you're mad here, it's a compliment.'

Directly above where Day-Lewis sits in an armchair next to the fireplace there is a picture frame tucked on to a bookshelf in front of an old Joan Armatrading album. It contains two photographs, one of his mother, taken in the late 1940s when she made an eye-catching film debut as Madeleine Bray alongside Sybil Thorndike in Nicholas Nickleby. Her son has inherited her blue-black hair and striking good looks, though his eyes are grey to her liquorice.

The other is of his father. There is something of him, too, in his son, perhaps more markedly as he gets older - a craggy, lined handsomeness that had Rebecca West remark of the poet laureate that he was 'like a Greek Apollo, with some irregularities set in to make him look not too bright and good for human nature's daily food'.

That cragginess adds power to Day-Lewis's performance in the second half of There Will Be Blood when Daniel Plainview discovers that sudden wealth has a heavy toll as he descends into paranoia and barbarity. The themes of oil, greed and unbridled capitalism in the film have a strongly contemporary echo, especially at a time when America and its allies are embroiled in a conflict in Iraq that many believe is all about oil. But they were not, he says, what attracted Day-Lewis to the part.

'It was Paul [writer-director Paul Thomas Anderson] really, in the form of his script,' he explains. 'I'd loved his films [Punch-Drunk Love, Magnolia, Boogie Nights]. And the idea had occurred to me that we might enjoy getting up to the same kind of mischief, but when this script came it really took me quite by surprise in the most wonderful way. The bag was packed once I'd read it.'

He makes it sound rather like a boy's own adventure and, although he turned 50 last year, there is something youthful about the tall, lean Day-Lewis. His long black hair has only the lightest dusting of grey. His two hooped gold earrings caused one visitor recently to describe him as looking like a bohemian pirate. And his Triumph T-shirt, worn with jeans (I'm not going to reveal the colour of his socks), goes with his love of motorbikes.

The parallels that can be drawn from Plainview's story must, I suggest, have given the script added pulling power. 'Paul's not unaware of what is going on in the world but our focus had to be a much narrower and more selfish one,' Day-Lewis replies. 'If you enter into the realm of trying to create a parable or cautionary tale, you've already strayed so far off course that you might as well stay in bed. So no, it was utterly and specifically that man in that story in that place at that time in America's social history.'

So what precisely was it about Plainview and his story that persuaded Day-Lewis to abandon Ireland and uproot his family for several months of life on a film set in Texas? He pauses. 'I wouldn't know what to say really.' The soft Irish lilt in his voice is as far removed as can be from the precise but hard-edged bark of Plainview. 'It is one of the blessings of that situation where you feel drawn irrevocably towards the discovery of a life as your own begins to recede behind you.' It is an arresting image for the acting process, and gets to the heart of the Day-Lewis method.

'Can I ask you about your method?' I venture. 'God help you,' Day-Lewis responds, but he says it not with exasperation - as when Harold Pinter is asked what his plays are about - but rather as one who clearly struggles to define it himself.

'It sounds so presumptuous to talk about it but I had a strong sense of the power of Paul's unconscious in his script and in his work. And it appealed very much to something in mine, and I never chose to define it or analyse it in any way whatsoever. He honestly told unblinkingly the story of one man's life from the first scene to this outrageous conclusion. I couldn't begin to imagine where some of that had come from because it didn't always appear to have a logic, and yet it appeared to me to have its own innate logic.'

That element of the unconscious crops up repeatedly in our conversation. And it remains, to the last, hovering in the air. To deconstruct it further might, Day-Lewis believes, somehow dissipate the magic. It is not that he is trying to be mysterious - that lifelong study in evasion - but rather that he himself finds the source of his own tremendous gift elusive. 'I've no idea what that transaction [involved in taking on a role] is all about,' he continues, 'or from where the need arises, but it is a response obviously to a very particular need at a very particular time, a need to express oneself in that way. I could dismember the script now and tell you all about its wonderful qualities but that wouldn't really tell the story of why I did it.'

Going on instinct can be a hit or miss business but in Day-Lewis's case it has served him well. He is one of the few major-league actors who has never taken on a duff role. He began acting early. After going to the local state primary school in south-east London he had struggled to adapt when sent to the private Sevenoaks School but eventually found a more congenial environment at Bedales, the progressive boarding school in Hampshire, where he joined his older sister. Drama and carpentry were twin passions. In his mother's Hampshire cottage is a beautiful circular dining table and chairs he made as a gift for her. He had the chance of developing this skill further with the offer of an apprenticeship when he left Bedales but opted instead to go to the Bristol Old Vic theatre school under the legendary Rudi Shelly. His early work was all on stage but in 1982 he had a small part in Gandhi. When My Beautiful Launderette and A Room with a View opened on the same night in New York in 1985 he made such an impact on the critics with two such different roles that he won a best supporting actor award.

After that he worked pretty constantly for the rest of the decade but took a three-year break after My Left Foot, returning in 1992 in The Last of the Mohicans, a huge box-office success. Since making The Boxer in 1997 the breaks have become longer. His sons were born in 1998 and 2002. In the latter year he made Gangs of New York and in 2005 The Ballad of Jack and Rose, written and directed by his wife, Rebecca, in a role that she had first tried to persuade him to take before they met.

'I can't re-examine work I did in the past with pride,' he says, resolutely resisting the invitation to flick through his back catalogue. 'I can only re-examine it with a kind of curiosity about the person who felt the need to do that work at that time, because you don't recognise yourself.'

He does, however, own up to one bad decision 'which I don't regret because the experience was probably one I learnt from. I was swayed by the hullabaloo surrounding The Unbearable Lightness of Being, which I was encouraged to believe every young actor was after. If I'd really shut out the din and looked at that script, I'd have known that I wasn't ready for that. I felt I was short-changing them somehow because I was missing the centre of it. It was sliding away from me.'

In the 1988 adaptation of Milan Kundera's novel set in the Prague Spring of 1968, Day-Lewis played Tomas, a Czech brain surgeon. 'It was,' he muses, 'something to do with language. The idea of speaking English with a Czech accent without actually speaking Czech meant it wasn't coming from anywhere - I knew that that kernel of truth that I need to have somewhere in a role would be missing. And apart from anything else, the exploration of sexuality in the film was just - well, I was in no way prepared for how that would feel. It was a mistake.'

The idea of learning Czech in order then to speak in English with a Czech accent would not, I suspect, enter into many actors' calculations when considering a role but another much discussed aspect of the Day-Lewis method is the preparation he puts into a role. He is reported, for instance, to have lived separately from his wife and children on the set of The Ballad of Jack and Rose. 'I did,' he says, a stickler for accuracy, 'but it was a token, symbolic separation which nevertheless to the imagination can have a powerful effect. I still saw my family every evening and spent the weekend with them but it would otherwise have been just too complicated for me to play the extremely conflicted life of Jack while in the presence of someone I was married to.'

Put that way, it sounds entirely reasonable. What then of the two days and nights in a prison cell without food or water that he endured to prepare to play the role of Gerry Conlon in In the Name of the Father (for which he earned the second of three Oscar nominations to date)? 'You have to learn,' he answers simply. 'You need to understand what it is like to be interrogated by three two-man teams over a period of two days. If an innocent man signs a confession, which pisses away his life, it is part of your responsibility to touch on why a human being would do that. So my curiosity leads me into those places. But I don't want to make too much of the details. They are just that - details. When you don't know from experience, or you can't explore through the imagination, you better do some sort of practical work that is at least going to stimulate the imagination, because finally the whole thing is just an act of imagination.'

He stops. 'It all sounds so ponderous and self-important. It's why I avoid talking about the way I work. But in avoiding it I seem only to have encouraged people to focus their fantasies about me in an ever more fantastical way on the details that are not at all at the centre of the work.'

There is, it is becoming clear, not madness but an attractive modesty about Day-Lewis. 'It wouldn't occur to me,' he explains, 'to talk about my work unless I was in an interview situation.' And indeed, during our previous meetings, he never has. 'I wouldn't see the point in trying to describe it in any way.'

The director Jim Sheridan, with whom Day-Lewis made My Left Foot, The Boxer and In the Name of the Father, once reported him as saying that he hated acting. 'I dare say I did when I said it,' he retorts good-naturedly. 'Who doesn't hate the thing that they most love? Acting is an impossibly illusive trade to ply, but the prevailing sense I have when I go to work is one of joy. It is always represented as a kind of self-flagellation for me. It couldn't be further from the truth.'

Since Day-Lewis seems in the mood to demolish some of the other 'mad bastard' stereotypes that surround him and his work, I try out a few others on him. For instance, there is the idea that, away from a film set, he retreats into another world that is as far removed from movie making as he can get. 'It is misleading,' he explains, 'to see my life in front of the camera and my life at home with my wife and children as two lives between which there is a schism. My life as it is away from movie set is a life where I follow my curiosity just as avidly as when I am working. It is with a very positive sense that I keep away from the work for a while. It has always seemed natural to me that that in turn should help me in the work that I do.'

So the Wicklow mountains, spread out magically in the winter sunshine outside the study window, aren't an escape but a source of inspiration. 'In a rural parish,' Day-Lewis explains, 'you become utterly unnoticeable. Or that's the impression I have. I couldn't work or get ready for a piece of work from a city base, from city life [though he and Miller do have a base in New York]. I need deep, deep quiet and a landscape too that I can be absorbed into. So much of the work is in the process of aimless rumination in which things may or may not take seed.'

Another of the Day-Lewis myths concerns what he calls the 'Hamlet experience'. In 1989 he was well into an extended run as Hamlet at the National Theatre when he walked off stage mid-performance and has never trod the boards since. It was reported that he had been unnerved when he felt he was talking to the ghost of his own father. 'It's not,' he recalls, 'that I appeared on stage one night and disappeared the next. I was working and living with that play for a year and a half of my life. And it's a weighty play to live with, so it didn't really surprise me that I got tired by the end.'

So tired, not haunted. Nevertheless, his feelings about theatre are mixed. He has been quoted in the past as being dismissive of the stage 'That's just me gobbing off again,' he laughs. There is, though, he admits, a problem that arises from his particular approach with the process of preparing for a role in the theatre compared with films. 'Theatre invites a nuts and bolts process to rehearsing in which all the actors are transparent to each other. For me, even if the truth I am looking for might be a specious one, I still need to believe in a kernel of truth. And I find it hard to do in a rehearsal situation where everyone is saying, "Are you going to do it like that?" It is distracting and deadly in the end to any discovery you might make. I'm never far away from a sense of potential absurdity of what I am doing, and maybe as I get older I have to work harder and harder to obliterate it. That's maybe why I seem to take it far too seriously.'

Another charge laid against Day-Lewis is that he has somehow turned his back on his home-grown film industry in favour of America. 'I am,' he acknowledges, 'rather surprised that I haven't made more stories about my own country but it is a mistake to suggest that the biggest influence on my life in terms of movies has been America. It was and remains Ken Loach and his whole body of work, not that I have ever worked with him. There is something unique and pure about the way he works, without a taint on it. His beliefs have remained unwavering since he made Cathy Come Home.'

One purist, you might conclude, admiring another. Day-Lewis is undeniably a film fanatic. He still loves, he says, wandering into a cinema and taking pot luck with whatever is on. There is something more accessible, he feels, about film than theatre. And he talks passionately about those whose work he most admires. I leave him inspired to re-examine the films of Montgomery Clift and Charles Laughton. When he talks about his own country, surely Day-Lewis must now mean Ireland? 'Yes, I do have dual citizenship, but I think of England as my country. I miss London very much but I couldn't live there because there came a time when I needed to be private and was forced to be public by the press. I couldn't deal with it.'

We're back where we began - on the wisdom of actors being public figures. Did he ever enjoy that side of his work? 'Initially it was invigorating. People suddenly wanted to hear my views on all manner of social problems. I was up for it but it palled very soon afterwards. It was not like real conversation where you listen and learn. It's hard to learn anything when you are talking about it. You only learn doing it. And if you are not learning, what's the point?'

His seriousness of purpose can, then, only be judged by its products. In the case of There Will Be Blood - a film for which it took two years to raise the money from sceptical financiers - Day-Lewis appears set to reap yet more accolades. The nominations and Oscar talk must feel like a kind of vindication. 'Well, there is some chance people may go and see it because we've been nominated before we become the losers,' Day-Lewis laughs. 'Being nominated is an ideal situation to be in. What is not helpful is being nominated and then not winning because then people think, "Oh Christ, they lost so I'll go and see something else instead."'

The key films

My Left Foot (1989)
Director: Jim Sheridan

To portray Christy Brown, the Irish artist who suffered from cerebral palsy and was only able to control his left foot, Daniel Day-Lewis asked to be wheeled around the set in his wheelchair, and crew members were required to spoonfeed him and lift him around. He damaged two ribs during filming from hunching in his wheelchair for weeks on end. He taught himself to paint using a knife held between his toes, and studied disabled patients at Sandymount School and Clinic in Dublin. Day-Lewis won a Best Actor Oscar.

The Last of the Mohicans (1992)
Dir: Michael Mann

Day-Lewis played Hawkeye, a British man adopted by Mohican warriors who is forced to fight during the French and Indian War of the mid-18th century. To get into shape for this particularly physical role, Day-Lewis underwent rigorous weight training during which he added 20lb of muscle to his body. Not content with that, he also learnt to live off the land and forest, as his character would have done, by spending six months learning how to camp, fish and skin animals. By the end of his training he had built himself a canoe. He also carried a Kentucky rifl e at all times during filming and learnt how to load and fire it while running.

In the Name of the Father (1993)
Dir: Jim Sheridan

For the part of Gerry Conlon, who was wrongly convicted of the Guildford pub bombing in 1974 and jailed for 15 years, Day-Lewis lived on prison rations to lose 30 lb and spent extended periods in the jail cell on set, while crew hurled abuse and cold water at him.

The Crucible (1996)
Dir : Nicholas Hytner

For this adaptation of the classic Arthur Miller play based on the Salem witch trials of 1692, Day-Lewis went back in time. He stayed on a Massachusetts island in the film set's replica village - without electricity or running water - planted fields with 17th- century tools, and built his character's house.

The Boxer (1997)
Dir: Jim Sheridan

Day-Lewis played a former IRA member and boxer just out of prison. He traine with former world champion Barry McGuigan, who said he could have been a professional: 'He was in the gym twice a day, seven days a week for nearly three years.' Injuries included a broken nose and a herniated disc in his lower back.

Gangs of New York (2002)
Dir: Martin Scorsese

To become 'Bill the Butcher', Day-Lewis hired circus performers to teach him how to throw daggers and trained as a butcher. He got pneumonia during shooting, initially refusing to have treatment or trade his coat for a warmer one.

Bharat Azad

· There Will Be Blood opens on 8 February