A shape-shifter on the stage

Aidan Gillen may not have a face you instantly recognise, but he can change remarkably for each character he plays. It's better like that, he tells Kate Holmquist.

He doesn't stop moving in the hotel armchair. He pulls his black Adidas jacket up over his ears as though he's hiding. Then pulls it down and tries to sit still, but then his hands are in his hair constantly, pulling it, bending it, working it like clay, until eventually it stands straight up, like the black comb of an exotic bird.

I keep looking for characters in Aidan Gillen's malleable face: for the seductively amoral Stuart in Queer As Folk, for councilman Tommy Carcetti in The Wire, for Edward in Frank McGuinness's play, Someone to Watch Over Me or Mick in Harold Pinter's The Caretaker, for which Gillen was nominated for a Tony award after a New York Times review said he had outshone his co-actors, Kyle McLachlan and Patrick Stewart, not because they were inadequate - they were great - but because Gillen's energy was amazing.

Still dancing in his chair, Gillen leans on the marble shelf beside him, looks into the mirror and seems to watch the room behind him more than himself, with that shock of black hair sticking up like it's electrified. I'm distracted towards the mirror in an attempt to see what he's seeing and when I turn back, I realise he's been watching me watching him.

"You're a shape-shifter, aren't you? You can totally change your appearance at will," I suggest.

He smiles, settling into his chair. "Yea, I guess so." I've got his attention.

"You like that, don't you? Being described as a shape-shifter?"

"You are living through other people's stories. This is your life. Some of your most intense moments in your life are while you are pretending to be someone else. Hopefully, you have some sort of life that's your own, but it's all the same thing, really."

"Do you know who you are?" I ask him.

"I have an idea. Working as an actor . . . You are getting deep into being somebody else and that becomes a large part of your personality. Then you could be somebody else again next week," he says.

He has deliberately avoided being type-cast. "If something works you get asked to do four or five things the same. I've made an effort for the next job always to be something different."

Before Christmas, Gillen finished shooting a nightmarish thriller in Barcelona with 29-year-old director Rigoberto Casteneda, Black Out. He plays "an Alpha male doctor with a dual life", who becomes trapped in a lift with two other characters, played by Amber Tamblyn and Arnie Hammer. The film's tagline is "hope is not on the way".

GILLEN HAS MADE nearly 30 films and has appeared in dozens of theatre productions, but hasn't registered on the romantic lead radar like previous Irish heart-throbs Gabriel Byrne, Aidan Quinn and Colin Farrell. In looks, he's most like Farrell, with a bit of Stephen Rea and Richard Dreyfus thrown in, but without Farrell's charismatic public persona. Gillen is a light-switch, turning various qualities of energy on and off - sometimes within the space of a minute. And unlike Farrell, Gillen's face can be so ordinary that you'd miss him in a crowd. Now aged 38 and with 20 years of professional acting behind him, Gillen says he's aware at being just the right age to start leading in the sort of Hollywood films that could take him from highly respected working actor to mega-stardom.

He's so hard to pin down, he doesn't even identify with being an "Irish" actor. "In New York, they think I'm English, and in England they think that I'm from Ireland. If people don't have a strong sense of where you are from, it's to your advantage. I've never tried to push any kind of image. I feel at home in a situation where nobody knows who you are and what you do. I prefer not being recognisable in the street, the less people think they know about you the better. What do they want to know about you for, anyway?" he asks.

The son of an architect, who died last year, and a nurse, Gillen grew up in Drumcondra and was an altar boy in Gardiner Street church. At 14, he became involved in the National Youth Theatre but he didn't see it as a career. By Leaving Cert year, he'd fallen foul of the Christian Brothers. "A month before the Leaving Cert, a teacher sent me out of the classroom and I lay down on the roof of the building and decided it was time to move on. I liked school, but I didn't want to do any more of it," he says.

Gillen's "university" education began with a year living in London "watching the video of every film ever made". Then in 1988 he was cast in Billy Roche's Handful of Stars and met Jenny Tupper, who ran the Bush Theatre. "I lived with her for two and a half years. She was very supportive and encouraging." There was no sexual intimacy between them but Gillen says he got slagged for being her "toy boy", which he wasn't. "There was always a glass of Friexnet and a cup of quails' eggs and stimulating company of theatre people - Simon Callow, Alan Rickman, Frank McGuinness, Cathy Bourke, Lindsay Duncan. Always stuff going on. One time I was dying in bed with chicken pox, which I got at the rather old age of 21, and I woke up to find Frances Barber spoon-feeding me coconut ice-cream, which was almost like a fever dream. It was nice to have somewhere to be where I could talk to people, instead of an empty room on my own."

The actor he most admires is Donal McCann and the Irish playwright he would most like to work with is Conor McPherson - and McGuinness again, if the opportunity arose. As for Irish film-makers, he can't name one up-and-coming director he'd like to see cast him, which probably says something about the state of the Irish film industry.

GILLEN IS IN Dublin to play Teach in David Mamet's American Buffalo at the Gate. "What interests me most is the simple and straightforward plot: three small-time hustlers, hungry people, attempting to execute a rare coin heist and their efforts to keep on top as it falls apart. But behind that there's the background of the consumer capitalist society that they believe in and are existing on the margins of, and sometimes their efforts are desperate and desperately funny. They're treating each other like junkyard dogs, yet they are human. They're like a family. I think the play's got a heart even if the background is pretty black . . . Living is pretty black."

For the past three years, working on The Wire, he has fallen a little in love with the city it portrays, Baltimore - which likes to call itself "charm city" even though it's an east-coast backwater with a murder rate that's one of the highest in the US. Baltimore has been celebrated by novelist Anne Tyler and film-makers Barry Levinson and John Waters, but for the US public it is journalist David Simon who has explored the "rotten core" of this city, through drama series based on his non-fiction books that read like thrillers. Simon's first venture was Homocide: Life on the Streets, followed by The Corner and now, The Wire, which is about to film its fifth and final season.

Gillen has a real affection for The Wire, which Newsday called "the greatest dramatic series ever produced for television". He speaks passionately about the series' concerns, particularly how the education system is failing boys who have to decide whether to make something of themselves through education or join "the life" of street crime. One of the main characters is a cop turned teacher who becomes personally involved in his students' lives.

I tell him that my mother was a teacher in an all-black school in Baltimore; she kept a mattress for her eight-year-olds to sleep on and gave them money to buy lunch, since they stayed up all night with mothers on drugs and needed sleep and food more than education. He's fascinated to hear that people were trying to save these kids 40 years ago, but says: "I think there's a feeling that things are turning round in Baltimore."

Once we make the Baltimore connection, Gillen becomes himself and starts naming places he's become attached to as he kills time during breaks in filming: the Charles Cinema, which plays every arthouse film on the planet, and the Charles Club, a louche bar recently redesigned by the same designer who has worked on Barry Levinson's films. I tell him I got my European cinema education, for what it's worth, in the Charles Cinema and ask him if he's been to the Block, a porn district so risque in a 1950s kind of way that it's like a time warp. He instantly responds: "The Block. And it's right there in the middle of everything, by city hall!"

Blaze Starr, a stripper whose spinning neon pasties are one of my enduring childhood memories, was practically patron saint of the city when I was growing up there. He's fascinated by this bit of Baltimore porn history. Then we exchange John Waters anecdotes and he tells me that, in Fell's Point, the nutty little shop where Divine held court still exists. The Poles still take pride in their blonde beehives and now call themselves "Huns", he says. "When I get back to Baltimore, I kind of feel, you know, 'home'."

The cast and crew of The Wire can be "a bit cliquish" and Gillen likes his own company, though every weekend he can he flies back to London to see his two young children. As we say goodbye on the street, he vows that he will get a mention of Blaze Starr into The Wire. I watch him pull the collar of his chunky sheepskin suede jacket up around his ears, pat down his hair to make himself innocuous, and disappear into a city where only his old friends recognise him.

American Buffalo previews at the Gate Theatre from Feb 8 and opens Feb 13

Role model

Recent theatre creditsinclude Someone Who'll Watch Over Me by Frank McGuinness, New Ambassadors Theatre, London; The Caretaker by Harold Pinter, Roundabout Theatre Company, New York, for which he received a Tony Nomination; David Hare's adaptation of Anton Chekhov's Platonov, Almeida Theatre, London; Shakespeare's The Tempest and JM Synge's Playboy of the Western World, also at the Almeida; Mojo by Jez Butterworth, Royal Court Theatre, London; Marvin's Room by Scott McPherson, Hampstead Theatre, London; The Water Engine by David Mamet, Hampstead Theatre, London; Billy Roche's Belfry and Handful of Stars, Bush Theatre, London.

Film and television creditsinclude Rigoberto Castaneda's film Blackout; seasons three and four of The Wire; Law and Order; Shanghai Knights; Dice; The Final Curtain; My Kingdom; Queer as Folk, for which he received a BAFTA nomination; The Low Down, for which he was voted Best Newcomer at the 2000 Edinburgh film festival; Buddy Boy; Some Mother's Son; Circle of Friends; Mojo; Killing Time; and Safe.