Firth ditches Darcy to play the coolly affable psychopath

He gained an army of female fans after he emerged dripping from a lake in the television version of Pride and Prejudice. And they envied Renée Zellweger as Bridget Jones when she was the subject of his gently amorous attentions on the big screen.

He gained an army of female fans after he emerged dripping from a lake in the television version of Pride and Prejudice. And they envied Renée Zellweger as Bridget Jones when she was the subject of his gently amorous attentions on the big screen.

But the new film from Britain's unassuming heartthrob Colin Firth is a radical change of image from the dashing - but essentially rather nice - romantic hero.

In Where The Truth Lies, premiered at the Cannes Film Festival yesterday, Firth plays Vince Collins, one half of a famous 1950s entertainment duo. He appears to be the affable English straight man to Kevin Bacon's brash American crooner, Lanny Morris.

But the public image hides the seedy underbelly of showbusiness excess. Instead of a clean-cut gentleman, Firth's character is violent, obsessed with sex and pops pills. He is seen smashing an audience member's head repeatedly on to the floor in what the director Atom Egoyan said was the most violent scene he had ever filmed. He also appears naked in a series of graphic sex scenes.

Speaking at the festival where the film is in competition for the top prize, the Palme d'Or, Firth insisted the role was "not usually a stretch for most actors" and that riding around in Derbyshire as Darcy, as he did in Pride and Prejudice, required more research. He said that in accepting the role, "I was not trying to manipulate the perception of me. I just go where I find it most interesting. I feel very comfortable in this sort of drama.

"The romantic comedy thing came relatively late in my life and took me by surprise. I'm still rather surprised I'm still so associated with it. Roles like this are to be excavated and found in the ancient archives of my career."

But he admitted that the contrast between "the perfect English gentleman drinking tea" and the Vince Collins off-stage who "beats the crap out of people" is startling.

He and co-star Bacon were required to collaborate to improvise on their routine and build a rapport. The murder mystery was shot in Hollywood, including Malibu Beach and the lot at Universal Studios where Egoyan admitted shooting was disrupted by tour groups.

Bacon said it was interesting as an actor to play another performer and explore some of the issues surrounding celebrity. "While I don't live a really extreme kind of celebrity lifestyle, I've certainly had plenty of experience of what that is over the years."

But he said: "I've played a lot of different kinds of people, it doesn't necessarily mean I've done all these things."

For Firth, who is married with two children, being an actor presented a paradoxical problem. "We're in the business of being very frank in one way, telling everybody about ourselves and exposing ourselves on screen," he said. But sanity, he added, depended on being protective of one's life as well. "We're completely condemned to be in that paradox always," he said.

His character in the film is so totally dependent on his showbusiness partner that he finds it difficult when their relationship is over and he slides into mediocrity and isolation.

Critics attending the screening wondered if the sex scenes would attract the attention of censors.

Robert Lantos, the producer, said they were not graphic but "steamy" and insisted there would be no problems. "It's a film for adults," he said.

It was also announced that Firth is shortly to co-star with Ben Kingsley in an action-adventure epic, The Last Legion. He will play a teacher who helps Romulus Augustus, a 12-year-old emperor of Rome, during the fall of the empire.