David Thewlis' performance as Johnny in Naked can be compared to that of Harvey Keitel in Bad Lieutenant -- it transcends the layman's comprehension of what acting is and what actors do.

The tall, rangy bloke in the pea green blazer sitting across from me in the hotel room is the same scraggly and bitter spitfire who strangled the hope out of me. Nihilist, cynic, ambulatory black hole, Johnny is the product of Thatcherite Social Darwinism, the apotheosis of an uncaring society, the ultimate antihero in Mike Leigh's latest and most disturbing examination of life in post- modern Britain.

Thewlis, 31, had a role in Leigh's last film, Life Is Sweet, so he's used to the director's method of working without scripts. Instead, Leigh spends months working individually with his actors creating character histories, complete with grandparents, parents, lifestyles, addresses and dark memories. Only then does he bring these phantoms and their actors together to improvise dialogue.

During the actual production, says Thewlis, the actors were improvising scenes on the day. "We fix exactly what's going to go on camera sometimes 15 or 20 minutes before we shoot it."

Frankly, I didn't believe him. How can you hope to achieve continuity if each take is improvised? An editor generally needs a few angles - - master shot, close-ups, etc. -- in order to create a stimulating sequence.

Thewlis is pleasantly exasperated. "It's not improvised on camera. It's like if you repeat what you just said and then I say, 'It's not improvised on camera,' and we do that again and again. It hasn't been written because it comes from improvisation, because conversation is improvisation. We know exactly what we're going to say and we don't change it from shot to shot."

Considering the length of some sequences, that's quite a feat of memory.

"Yeah," says Thewlis, grinning, "it was."

Naked is an exhausting, harrowing journey through London's mean streets. The film follows Johnny's destitute intellectual and his parallel opposite, the rich and facile Jeremy. Together, Johnny in his dingy overcoat and Jeremy in his midnight Porsche, they are two black vectors piercing the lives of women they know or come across.

Like many pivotal works of art, like Nabokov's Lolita or Burgess' A Clockwork Orange, its brutal honesty will set off alarm bells amongst the censorious. Watching the film you feel the shock of the new, the sense of limits being stretched.

Thewlis felt it during the production. But while it was then apparent that Leigh was investigating sexual violence, not until after seeing the film for the first time did Thewlis appreciate the scope or the malevolent shadow of his alter-ego, Jeremy. Unless they appeared in scenes together, the actors had no idea of goings-on elsewhere in the film's world. "If you know something about the character, it prejudices the improvisation. Our characters were top secret."

Thewlis wasn't allowed to tell his wife (an actress herself) for fear that she might tell someone else and the character's essence be leaked to one of his Naked co-stars. "That way," says Thewlis, "the reactions are real."

"I had to trust that Mike was not making a film that would go against my own principles. I know his politics, I know that he is responsible for some of the most positive portrayals of women in British film, I know he's not a misogynist. I was playing a character with a history of violence towards women and so I had to maintain the integrity of that character."

Johnny, says Thewlis, "is based on real people, real case studies. Women have the potential to render him vulnerable and inadequate. He blames them for his sexual arousal and the potential of that to humiliate him. His humiliation expresses itself in violence. It comes out of fear and frustration. He thinks he understands everything but obviously he doesn't understand women."

At the same time, Johnny invites violence from men, asks them for it. At one point in Naked, Johnny follows and pesters a punk posterer for hours until the exasperated man kicks Johnny in the bollocks and drives off. Finally, after being beaten senseless by a gang of roving bullyboys, Johnny seems on the verge of self-discovery. "You may think I'm full of shit," he says. "I'm not. I've had it kicked out of me."

It's just this sort of line, this dark wit, which forces Johnny into your subconscious, to be rediscovered long after the film is over.

Johnny has won Thewlis the Best Actor award at last year's Cannes Film Festival and from the New York Critics Circle; he is touted for an Academy Award nomination.

As a result of Leigh's improvisation technique, Thewlis effectively wrote himself out of his role as Jane Horrick's boyfriend in Life Is Sweet. He laughs, recalling how it happened.

"Jane says, 'You want to go upstairs and have a fuck?' and I say, 'No, I want to have a conversation,' and she says, 'Fuck off then,' and I say, 'All right, I'll go,' and as I was walking out the door, I said to myself, 'Aw, fuck, I've just written myself out of the film.'

"That's what happened to the guy who put up the posters. After he kicked me in the balls and drove off, Mike said 'Well, he's out of the movie.'

"I've worked with Mike before -- I know the trick."