We'll never know if Tupac Shakur could have been a movie star. But Gridlock'd ,which he completed two months before his death in a still-unsolved shooting incident last September, proves that he had the dynamism and flair of a major screen actor. Written and directed by Vondie Curtis Hall, the movie, set in the squalid backstreets of Detroit, is a vibrantly gritty lower-depths comedy, a tale of hapless junkie thieves, played by Shakur and Tim Roth, who bum around the city like a couple of alley cats, torn between their desire to score and their desperation to kick. As Gridlock'd goes on, the two have run-ins with cops, drug dealers, gangsters, and a welfare bureaucracy so rusty and sclerotic you'd call it Kafkaesque were there any real design to it. Desperate to get into rehab, they're tossed from one ugly fluorescent-lit government office to the next, a comedy of urban errors that escalates in insanity when the police mistake them for killers. Gridlock'd doesn't have the imaginative vision of a movie like Trainspotting , yet it's more literally true to the haphazard torpor of the junkie life than anything we've seen on screen since Drugstore Cowboy .

Making his debut as a filmmaker, Curtis Hall, an actor himself ( Chicago Hope, William Shakespeare's Romeo & Juliet ), shows a gift for back talk and confrontation, for the hardscrabble comedy of urban decay, and for an electric storytelling style that sometimes shades off into B-movie glibness. Gridlock'd opens with a jolt: On New Year's Eve, Stretch (Roth) and Spoon (Shakur), holed up in their garbage-strewn loft, discover that Cookie (Thandie Newton), the slinky beauty they're with, has overdosed herself into a coma. They drag her to the hospital and go off into the night, numb but shaken, determined to change their lives. As it turns out, the three are partners, members of a bohemian spoken-word/jazz trio. They play together, live together, sometimes even sleep together. But from the moment Stretch and Spoon reach the street, with Cookie lying in limbo, the two men are such raggedly ill-disciplined lowlifes — at times, they seem like homeless derelicts — that I never really bought them as quasi-professional musicians.

Still, if the setup is facile, what follows has a bombed-out authenticity. Curtis Hall has caught the bottom-feeder enervation of heroin addiction — the fact that most of it consists of shooting up, nodding out, talking about scoring, going out to score, and then starting the whole business over again. The fun of the movie lies in the way that Roth and Shakur manage to seem feverishly alive amid all this manic-depressive running in place. More than any other contemporary British actor, Roth adores playing scuzz-ball Americans. He makes the dense, gawky Stretch defiantly myopic, a hustler who can't focus on anything but what's directly in front of him. As for Shakur, with his morose elegance and beautiful liquid eyes, he lends Spoon a tremor of sorrow, projecting a supple emotionalism almost entirely at odds with his gangsta-prince image — the hell-bent rap nihilist so intoxicated by the ''thug life'' that, in the end, he literally appeared to be courting his own murder. To a true thug, of course, vulnerability is unthinkable; it makes you look soft. But as every good actor since Brando has understood, vulnerability can lure a movie audience right into your soul. Shakur had it to spare, along with many other qualities: wit, sexiness, a self-involvement poetic in its intensity. His tragedy may ultimately be that he didn't believe his humanity mattered.