Red hot: The irresistible rise of Damian Lewis

Damian Lewis is an intense chap, capable of conveying a huge range of emotions with the smallest gesture. He's hotly tipped for an Oscar for his new film. And he's a real gent. Just don't call him posh, whatever you do. Interview by Liz Hoggard

"Ask him about that intense thing he does with his eyes," a female journalist suggested when she heard I was interviewing the actor Damian Lewis. What's striking about Lewis is how much he manages to convey by doing so very little. There is stillness about him on screen, a faraway look that can evoke anger or desire or - if you saw his rollicking performance as Benedict in BBC1's modern-day version of Much Ado about Nothing - sheer hilarity.

The press love to brand Lewis as an arrogant posh boy. Like David Cameron, he went to Eton. But, among his generation of actors, no one does grief and repressed emotion so well. In Spielberg's Second World War epic, Band of Brothers, he played an American soldier facing up to fear with a quiet certainty (it won him a Golden Globe nomination). He was the bewildered newlywed who doesn't understand why his marriage is falling apart in Hearts and Bones. And in the remake of The Forsyte Saga, he did the unthinkable - making the brutal Soames sympathetic.

For several years now, 35-year-old Lewis has been a successful actor on the verge of becoming a major star. Unlike Ewan McGregor or Joseph Fiennes, his contemporaries at London's Guildhall drama school, you might still walk past him in the street. But all that should change with the release of his new film Keane: his performance is already sparking Oscar rumours in the States.

Lewis plays a man battling mental collapse after the disappearance of his eight-year-old daughter. He spends his days, feverish and unshaven, staked out at New York's Bus Terminal, from where he believes she was abducted. It's a fantastically unvain performance from an actor we're more used to seeing modelling clothes in glossy, society magazines. Or impersonating Elvis on stage at the Berkeley Square Summer Ball. But then Lewis is a chameleon performer.

His screen icons are Steve McQueen and Gary Cooper, stars who had a natural economy with words. "If you set up an intensity and a stillness to someone, you only have to show a flicker of a smile and it will show volumes. " He also loves the way De Niro actively listens. "He does it brilliantly. It's his listening that gives him his mercurial quality. It shows a certain humility."

The same could be said of Lewis, although he unapologetically "enjoys frivolity". When I arrive for the interview he and his publicist are wading through a heap of party invitations; Lewis himself ironically brandishes the VIP discount card Versace have just sent him. "Not quite my type of clothes," he laughs, dressed down today in chinos and a leather jacket.

He takes his work seriously. "You can't do something that is morally vacuous or dysfunctional and then write it off saying, 'It wasn't my film, I was just doing a job in it'." But he's no snob. After The Forsyte Saga, he shot Dreamcatcher, an adaptation of the Stephen King bestseller, playing a man whose body is invaded by an alien. "If you only do issue-based drama, you can become a boring wanker," he warns. "Fluff is good, fluff has its place. You do your 'passion projects', as they call them in the States, but I'm really happy to make a film about aliens exploding out of people's bottoms."

In contrast, Keane is a little art-house film directed by Lodge Kerrigan. It's not always an easy watch, but it is utterly compelling. Keane spends his days self-medicating with drugs and alcohol, but we sense he was once a professional with a good life.

"Lodge is an uncompromising film-maker," says Lewis. "The first 35 minutes of the film are quite suffocating for the viewer because there is no great exploration of Keane's illness or his grief. He may even be entirely delusional, so that plays on the audience's mind: 'Did the daughter ever exist?' It's clear that he has acute anxiety and panic, and paranoia - all symptoms of schizophrenia. Although grieving in a rational way he is also clearly undergoing some sort of mental breakdown."

Lodge spotted Lewis in Band of Brothers and sent him the script for Keane. He flew over to London and the two men hung out together to find out if they were "compatible".

In fact Lewis has experience of mental illness (his first grown-up girlfriend had bipolar disorder). "There is a responsibility to get these things right if you're being asked to portray illness or mental disintegration or grief in as naturalistic a way as Lodge wanted to shoot the film." Both he and the director wanted to avoid a grandstanding performance. "Anything sensationalist would have stuck out like a sore thumb: I didn't want to do, you know, the 'greatest hits' of mental tics."

He spent time meeting people with mental illness and watching documentary footage. "The thing the film illustrates is how easy it is for any of us healthy, functioning middle-class types to tip over - it's five short steps to welfare cheques and homeless nights and drug abuse, and before you know it you've slipped right through the cracks in society."

Filming has changed him, he admits. "It's very easy to walk round, steer clear of, patronise... show animosity to the wino on the street, the guy or woman with needle tracks up their arm. I've lived in Camden for five years and your first response is: 'Go and do it somewhere else', which is completely unhelpful. We're always told, 'Don't give to people begging because you'll only feed the next fix', which is clearly what happens, but there is an act of some compassion just by giving the money. The only real answer," he says, slapping the table, "is to devote your time to a charity and do it through a formal system. Doing the film taught me a sense of sympathy and understanding that I perhaps have been slow to have before."

Lewis tends to play conflicted characters - Jeffrey Archer in Guy Jenkins' Jeffrey Archer: The Truth, or the Gatsby-esque inventor in Stephen Poliakoff's Friends and Crocodiles. "Dramatically it's always more interesting to conceal rather than reveal things." But also there are very few red-headed male leads. It's one of acting's sillier hang-ups.

When I interviewed him just before his performance in the National Theatre's production of Ibsen's Pillars of the Community, Lewis told me, "The treatment of people with red hair is inexplicable. Let's not even go there, it's so boring. I've tried to explain it cogently and rationally and objectively, but every time I do I just feed the argument. I don't know where it all comes from, but it certainly exists. In the future, I've promised myself to look into the history of it. But until then, I've sworn not to say any more on the subject."

Directors are missing a trick. Lewis can be powerfully erotic on screen. Tall and athletic, he looks like a man who instinctively understands why people should wish to look at him. His face is sculpted, handsome - only that tight line of a mouth allows him to play more offbeat characters. A cricket ball broke his nose as a child, so he couldn't breath through it. "Before I had it operated on I used to stand on stage with my mouth slightly open. Perhaps it made me look a little gormless."

Lewis was born in London in 1971 to an upper-middle-class family. His mother was an actress (later on the board of the Royal Court theatre); his father made his fortune as an insurance broker. "We come from quite a loud, garrulous, curious, challenging family. At Sunday lunches the volume levels were cranked right up. If you didn't get your point out, saying it loudly, quickly and succinctly, you didn't get it in, so there was a lot of confidence bred from that."

When he was eight he was sent to boarding school in Sussex. "Each summer we staged a Gilbert and Sullivan operetta. I used to sing the solos. I had a sweet treble voice." From there he moved to Eton. Age 16, he formed his own theatre company and put on a production of Nicholas Nickleby.

It was a blissful time, but he acknowledges, "when you go to boarding school at an early age, you learn to cope very quickly with your environment. That can create extraordinary social dexterity, but it can also leave you rather emotionally arrested, even if you appear superficially sophisticated and poised. If someone sees through that to who you really are, then those are... interesting moments." It's significant that he chose to go into acting, which he describes as "resolutely un-old-school-tie-ish. There's nothing 'inherited' about being an actor. You sink or swim."

He won a place at the Guildhall School of Music and Drama in London and later joined the RSC at Stratford for two years. Then came TV roles in Peter Kosminsky's award-winning Warriors and Hearts and Bones. His big break was the call from Spielberg and Hanks, who'd seen him playing Laertes in the Ralph Fiennes' production of Hamlet on Broadway. Lewis read for the part of Major Richard Winters, the commanding officer of the young paratroopers in Band of Brothers, and, within 48 hours, was in Los Angeles.

Like most Etonians, Lewis has exquisite manners. "It might sound smarmy but I enjoy nothing more than allowing a woman of an older generation through the door first, or getting up and offering her a seat. And I always cross the street late at night if I sense my presence is making a woman nervous. I don't care if anyone finds it quaint. In fact if I heard anyone suggest that it was patronising, I would find it hard not to laugh in their face."

As a teenager he was part of the generation growing up with feminism. " I remember feeling this overwhelming sense that one had to be sensitive to female preoccupations. Which is good and a natural part of one's education, but there was a sense that we were all trying too hard to find out exactly how to be 'new men'. Meanwhile, feminism was going through what many feminists would agree was a bit of a wrong turning: there was this idea that you had to behave like a man and be a ball-breaker to be empowered. Whereas today I think women realise they can use their own femininity, their own womanliness even, in a very powerful way."

For a few years he had a reputation as a flirt, dating the actresses Kristin Davis and Sophia Myles. Much was made of the fact that his ex-girlfriend Katie Razzall, a reporter for Channel 4 News, married his best friend, the actor and old Etonian, Oliver Milburn. Lewis attended the wedding with no apparent regrets.

Then he met the actress Helen McCrory when they were both cast in the Almeida's ill-fated 2004 production of the verse play Five Gold Rings. The play closed early, but their relationship blossomed. Earlier this year they announced their engagement and McCrory is due to give birth to their first child.

The couple are very private, but Lewis does say touchingly, "I think Helen is the best actress of her generation, but I'm biased. She's got a very rare ability to play leading women as total character parts."

His next roles are a long way from Keane. He stars opposite Connie Nielson in the political thriller The Situationist, set in Iraq, and he has just shot his brother Gareth's Welsh comedy, The Baker, where he plays a sensitive hitman who becomes disillusioned with his profession.

Lewis can be hard work - while he got rave reviews for his performance in Pillars of the Community at the National, rumours of starry behaviour drifted back. But in person, he is not what you expect at all. Thoughtful, self-deprecating, he worries about spending too much time in a fantasy world. "Why do you think so many actors are only half-developed people? It's very easy when you're a young actor to have these intense, explosive friendships for short periods of time, because you can control what's shown of you. Then you go on to your next job and reinvent yourself again. I think it's important to find something constant." One senses the influence of McCrory.

He openly admits he got himself into a state of anxiety making Keane. " I found if I looked behind myself enough times, and found people staring at me, it made me feel uncomfortable. I did slightly scare myself. Mind you, I talk to myself anyway," he laughs.

Lewis has known grief. His mother died in a car accident in India in 2001. "My mother's death is the single most important thing that's happened to me in my life." Did he access her loss for Keane? "It's completely different, that's what I mean when I say that, for me, acting comes from the imagination," he says firmly. "It's just my mind applied to the script and I imagine the rest. But," he adds softening, "if you've had tragedy in your life, you need to assume it has affected you in subliminal ways, and that will inform the way you judge your next performance. You might just get to a point in your life where you think: 'Life's too damn serious and too important to be doing fluff'."

As for Eton, he thinks the reputation for poshness is misleading. "The ruling toffs of old have now been integrated so much into the very middle-class professions of the media. My generation are the first to move into acting and film and I think - maybe it's naïve - that's helped lessen any prejudice towards Eton."

Lunch finished, Lewis prepares to depart. "Gosh, that was quite serious, " he says nervously. He needn't worry. Killing off the party boy image is a good move.

'Keane' is released on 22 September