It’s been almost two decades since the raucous Scottish movie Trainspotting crashed onto the scene; few films since have boasted such a raw mix of dark humor and unflinching reality. Adapted from a book by the same author, Filth has arrived with an identical swagger. Here is your next cult classic.

The movie industry had declared this Irvine Welsh novel to be “unfilmable.” The story is too dark, the plot too twisted, and the main character far too grotesque. But James McAvoy, the star of X-Men and Atonement, has pulled off a performance so searing and intense that he manages to imbue this drug-addicted, racist sociopath with just enough vulnerability to make the movie work.

McAvoy plays Bruce Robertson, a crooked cop who is happy to threaten and blackmail the criminals he is pursuing and hell-bent on doing much worse to the colleagues that might stand in the way of a promotion. Asked why he chose to become a cop, he tells a friend: “Police oppression, brother.” And did he want to stamp it out from the inside? “No, I wanted to be a part of it,” he answers.

The drug- and alcohol-fueled excesses build to a dizzying climax that leaves you reeling. As I staggered out of the screening, the director, Jon S. Baird, was waiting for me at the Soho Hotel in Central London. “You feel like you’ve been punched?” he said, smiling. “Nobody wants to come out of the cinema and go: ‘Oh, it was OK.’ You want to come out and say ‘Oh, fuck, that was awful!’ or ‘Gee, that was great!’’”

The film certainly makes an impact, and not everyone will enjoy the sight of McAvoy’s depraved detective inhaling mountains of cocaine with a younger officer played by Jamie Bell, who is more familiar as the angelic star of movies like Billy Elliot. Their escapades, by turns sexual, violent, and threatening, can make for uncomfortable viewing.

Lighter moments are provided by a great supporting cast which is led by Oscar-winner Jim Broadbent, who plays an eerie psychiatrist like a demented version of the therapist in The King’s Speech. Eddie Marsan, a British character actor, is superb as a sweet, nervous friend. He and McAvoy embark on debauched trip to Germany which unfolds manically to the soundtrack of “99 Red Balloons.”

Comic turns aside, it’s the intensity of McAvoy’s performance that sets this movie apart. “It’s mad, right?” said Baird, who wrote and directed the movie. “It’s amazing. I think this is his career-best turn. He’s in every scene, of course, but with that comes a huge pressure to take a character off the page, who is sometimes changing within a minute in the film. Within a scene, he’s going from aggressive to vulnerable to insane, so I just think it will show everybody how good he is.”

The character is pitched somewhere between Malcolm McDowell in A Clockwork Orange and Jack Nicholson in One Flew Over the Cuckoo's Nest. Baird thinks McAvoy’s performance should now be considered in that league. “I never wanted to compare it to particular actors, but I certainly think, in years to come, people will look back at this and say, ‘Wow, that was one of the big ones.’”

The Scottish-born actor hadn’t originally been considered for the role by Welsh and Baird, who had assumed he was from a comfortable suburban family. “James’s agent said, ‘Are you thinking about James?’ And we said, ‘Not straight off the top of our heads.’ But we met him here, in this hotel actually, and he connected so much with the character—he’s a lot more intense and edgy. He’s had a lot more of a challenging upbringing than I had imagined. As soon as he left the room, myself and Irvine were like ‘High five! We’ve got Bruce!’ We met him at 10 o’clock, and by 2 o’clock he had the part.”

That was 2011. By then Baird had been working on the project for three years. After writing the screenplay, he was unable to secure the backing of a major studio, but he eventually pulled together enough money for a low-budget shoot. McAvoy said he took a producer’s credit because they couldn’t afford to pay his fee.

Securing a U.S. distributor for a movie with so much violence, sex, and drugs has also proved difficult. Trainspotting had to be reedited and partially redubbed for its American release back in 1996. “There are a lot of companies playing safe at the moment,” Baird said. “We haven’t had a lot of challenging stuff for quite a while in the U.S., and I’m hoping that this is the first breakout—and we can make movies like we used to make.”

An American distribution deal is expected to be announced soon. But the film has already opened to huge crowds in Scotland, where it took more than double Trainspotting’s opening weekend. Early rave reviews suggest box office takings will also be high when it opens in the rest of Britain on Friday.

Audiences might be left stunned by some of the scenes, but, certainly, no one is going to walk out of the theater saying: “Oh, it was OK.”