The subtlest of movie stars takes on arguably his toughest role: a paedophile battling the demons within. Kevin Bacon tells Michael Dwyer about the difficulty of playing such a part, as well as the challenges and rewards of working with his accomplished wife

Few actors are as underestimated as Kevin Bacon, perhaps because his performances are so natural and unshowy that they seem effortless. Despite the quality of his work in more than 50 movies, Bacon has never been nominated for an Academy Award.

Last year, Bacon's Mystic River co-stars, Sean Penn and Tim Robbins, both received Oscars, but Bacon didn't make the shortlist, and he has been overlooked again this year, even though The Woodsman features his finest performance to date. In the film he etches a complex and compelling picture of a paedophile trying to readjust to life as an outcast in the outside world when he is released from prison under supervised parole after serving 12 years for child molestation.

It's an effectively understated and subtle performance, yet Bacon strikes a strong presence throughout the movie. "It took me a while to learn how to act like that, to get to a place where you're trying to get somewhere more internal," he said when we met in London recently. "I felt that it's an internalised piece of writing, so it needed that kind of performance. I worked a lot with the director on trying to take out lines - to say less but do more without relying too much on dialogue.

"It was a question of taking all this stuff about him and his life and history and his shame, mostly his shame, and bringing that mindset to the set every day. It sucks. It's not a nice place to go every day. There was no day when I could take a break from it, even on days when I had no dialogue - what I called bus days, when they would shoot me travelling around on busses all day and I wouldn't say a word. But even looking out the window of the bus, I had to carry all his pain with me. There were no pleasant days on that shoot."

The tone of The Woodsman is in sync with Bacon's performance, quiet and measured, and it never attempts to sensationalise the difficult material at its core.

"As much as I can, I want people to know that they are not going into a movie to watch a sex offence," he says. "There is nothing voyeuristic about the film, nothing sensationalised and nothing that could be interpreted in the slightest way as titillating. That's not something I would want to see, or something that I would be willing to do."

The most unsettling scene comes in a creepily powerful encounter between the paedophile and an 11-year-old girl in a park, and not just because it raises the fear that he will re-offend, but also as it necessarily involved getting a girl of around that age to play such a scene.

"I can tell you that, when I read the script and I came to that specific moment in the story, I had an idea how to play that scene," Bacon says. "I felt that if I could make that scene work, everything else could spread out from there. I saw it as the moment where this guy is on the edge of an abyss and you want to pull him back. The scene itself went through a lot of re-writes to get it right, and it's actually shorter than the scene that we shot, but I just think it works.

"It was a hard day. It was technically hard because we were out in the park and there were a lot of aeroplanes flying overhead and there was this knucklehead with an electric scooter he insisted on riding in circles around the park. That really screwed up the sound and it was really distracting, but the little girl was fantastic.

"I had met her and her mother and rehearsed with them. They are both very sophisticated and the girl understood exactly what the movie was about. I made it absolutely clear to her that I was not this guy, that I was an actor and she was an actress playing this part and the part was not her."

The screenplay, by young first-time director Nicole Kassell and playwright Steven Fechter, is so skilfully structured that it disguises its origins as a stage play. "Plays don't move like films do," Bacon observes. "And conversely, if you've ever seen TV footage of a scene from a play, it doesn't work because everyone seems to be screaming and exaggerated. It just looks awful even though it could be one of the best pieces of theatre if you saw it done on a stage."

Bacon came across the project under unlikely circumstances. He and his actress-wife, Kyra Sedgwick, were on their annual Christmas holiday in the West Indies when a man approached Bacon on the beach.

"He had nothing to do with the movie business. I think he was involved with shopping malls or real estate, and he said he had seen the script of this play he was thinking of investing in. He asked me if I would do him a favour and read it.

"Normally, I wouldn't say yes, because that's not how you usually get a script and there are all sorts of legal implications about getting one this way. But I asked him to send me the script and I read it when we got back home to New York in the first week of January. A year later we were in Sundance with the finished film."

In The Woodsman, Bacon's character finds work at a lumber yard, and he shuns friendships at work until a plain-speaking forklift driver (played by Sedgwick) breaks through his defences.

"She's the best actress I know," he says proudly. "When I was discussing the casting, I could think of very few people who could pull off that part - to be believable as a woman working in a lumber yard and to be so damaged and beautiful and sexy at the same time.

"But Kyra was very concerned with this kind of taboo around married couples making movies together. In Hollywood, that's usually regarded as a big negative. So it took a lot of convincing to get her to do the picture. Then, just as we were about to shoot it, there was this big article in the New York Times and it was called something like 'Why Couples in Movies Don't Work'. Kyra said, 'That's it, I'm out of here', but it was too late at that stage to get anyone else, so she stayed on.

"It's funny, because usually when you're making a movie you get introduced to some actor you've never met before and suddenly you have to invest all this feeling in forming a relationship with this complete stranger. That's part of our job, to fake that. With Kyra and me, it was the opposite, because we have to appear to be meeting for the very first time. We had to take away all we knew about each other after 16 years of marriage. That's a hard thing to get rid of, just like that."

As so often happens with thespians who marry, Bacon and Sedgwick met on a film set, when, in 1988, they were cast opposite each other in a critically acclaimed PBS TV film of Lanford Wilson's play Lemon Sky. When Bacon turned director in 1996 with Losing Chase, he cast Sedgwick with Helen Mirren and Beau Bridges in the leading roles. She also stars in his second outing as a director with Loverboy, which had its world première at the Sundance Film Festival last month.

"As her director," Bacon says, "I have to be gentle and supportive, which, she says I forget sometimes and I show more consideration towards the other members of the cast. I like directing her.

"She's fantastic in Loverboy. She plays this woman with a burning desire to have a child. That's all she wants. She's not interested in men, only to the extent that one of them gives her a child, and she goes on this quest for pregnancy. Her son is born and they have this incredible relationship in this insular world the two of them live in - until he decides he wants to see the rest of the world and she is so possessive that she begins to unravel."

The movie is a family affair in that Bacon himself is in it, their daughter Sosie Ruth plays the Sedgwick character as a girl in the flashbacks, their son Travis has a small part, and their dog is there, too. The cast also features Blair Brown, Matt Dillon, Oliver Platt, Campbell Scott, Sandra Bullock and Marisa Tomei.

Now 46, Bacon has come a very long way from his 1978 début down the credits of National Lampoon's Animal House, going on to demonstrate his range in movies as diverse as Diner, Footloose, Criminal Law, The Big Picture, JFK, The River Wild, Apollo 13, Murder in the First and Wild Things.

Along with The Woodsman and Loverboy, he will be seen in three other movies due for release this year: Beauty Shop ("I worked for six days on it; it's a Queen Latifah movie"); Cavedweller ("I just have a tiny part. Kyra has the leading role and she produced it. I just showed up and did a few scenes, although she cut some of them."); and the eagerly waited new Atom Egoyan film, Where the Truth Lies, with Colin Firth.

"We play a musical comedy team," Bacon explains. "We sort of cloned the act from everything that was happening in the '40s and '50s. It's not based on anybody specific, so we had the opportunity to create this imaginary act, and that played to our strengths, I think. In the act, Colin is this buttoned-down public school boy and I am this Elvis-y wiseass American rocker. We're an odd couple.

"The film is more concerned with what happens after a girl is murdered, and our characters may or may not have been involved, and then this young journalist played by Alison Lohman tries to uncover the truth in the '70s after we've split up as an act. The film moves back and forwards between the '50s and the '70s.

"Atom wrote the screenplay, which is based on a book, and the film has his signature on it, although I would like to think it is a form of a departure for him because there is something decidedly American about the film."

With such a prolific output, it will be even easier to play the amusing movie connections game that is Six Degrees of Kevin Bacon. "I think that's fun," he says. "When it first came around, I thought maybe it was a joke at my expense. That will be my epitaph: 'He won no Oscars, but at least he had a game named after him'."

The Woodsman opens at the IFI and UGC in Dublin next Friday