Oh, Mr Darcy     Colin Firth gets rumpy pumpy with Kevin Bacon and a maid.

The unthinkable has happened: British actor Colin Firth is talking about "shagging". Mr Darcy, shagging?

The heart-throb of millions of female viewers of the BBC's Pride and Prejudice and the Bridget Jones's Diary movies is expounding on the explicit sex scenes in his latest movie, Where the Truth Lies.

In Atom Egoyan's film we see plenty of Firth, who first sent a wave of hysteria across the globe as he emerged from a lake in a dripping wet shirt and breeches.

In Where the Truth Lies, adapted from Rupert Holmes's noir crime thriller, Firth plays the pill-popping, sex-driven Vince Collins, one-half of America's hottest showbiz musical comedy partnership of the 1950s. In one scene, Firth has a threesome with fellow comic Lanny Morris (Kevin Bacon) and a hotel maid (Rachel Blanchard), who is later found dead in their hotel suite.

"Safety in numbers," Firth says. "Actually, I was saved by Kevin's butt." He grins across the table at Bacon, so renowned for his cinematic bed forays that detainees on a US witness-protection programme complained online "that his dangling member gets into far too many movies. Somebody should talk to him about it."

"What do you mean?" Bacon says. "You showed up late that week after I'd done most of the hard work."

Firth: "I hadn't been filming the week that some solid shagging took place between Kevin and various women, so by the time I showed up there was no interest at all. The crew were so sick of the sight of his butt and mine offered nothing new. People make a lot of the sexual thing but that's really only one more weird thing we get to do."

Egoyan's film is a far stretch from his last one, Ararat, about the genocide of Armenians. The director's mandate was that the sex be sensual and unbridled.

"A role like that usually isn't a huge stretch for most actors," Firth says, who admits it was a refreshing change from the recent spate of romantic comedies. Was it an escape from typecasting for the theatre-trained thespian?

"Not necessarily," Firth says.

"They came to me quite late in my career and are fun, but the appeal was the character's inherent darkness and, of course, the opportunity to work with Atom."

Firth and Bacon's comic duo are loosely modelled on the lives of Dean Martin and Jerry Lewis. Egoyan suggested Firth aim for a mix of David Niven and Rex Harrison. The story moves between the '70s, when a young investigative reporter attempts to ferret out the repercussions of the murder on the duo's private and professional lives, and the subsequent break-up of their long-standing partnership.

Where the Truth Lies focuses on a complex exploration of celebritydom's underbelly. The film also gives the actors a canvas to improvise their own comedy and verbal jousts as well as sing.

Amend that to singing for one.

Bacon claims Firth hired a singing coach, only to be told his forte lay in the verbal thrusts.

For Firth, the drawcard lay in his screen persona's dark psyche.

"Vince is a very bleak character to portray. Playing him was a real stare into the abyss, actually. To desperately need your celebrity fix and yet have it as part of your burden must be a kind of hell.

"Rather than just playing a psychopath or mass murderer, it was interesting to play someone who is apparently what you expect me to be, and then take off the mask to reveal something darker."

Firth adds that depression is one of the least socially permissible things for human beings. "It is still considered very antisocial and shameful.

It used to be sex. But that's still a very private matter. Loneliness, fear and insecurity are much more so."