Scattered around his Soho flat are some of Kieran Culkin's favourite things: a mechanical toy hamster that wears a sombrero and sings salsa music; a two dollar bill; a plastic figurine of Panthro, the kung-fu crusader from the 80s cartoon series Thundercats. "He's totally my favourite character," says Culkin, manipulating him into a few heroic poses. At 20, the younger brother of Macaulay is defiantly adolescent, his chin bleeding from a shaving accident, his fingers dripping satsuma juice on to the white sofa ("it's not my sofa, whaddo I care?"), all the while avoiding eye contact through strategic deployment of his fringe.

But it is Kieran who, of the six Culkin kids, is making the steadiest progress towards a successful adult acting career. Last week he opened at the Garrick Theatre in Kenneth Lonergan's play This Is Our Youth, a slick and witty account of 1980s New York through the eyes of three slacker adolescents. His profile in Britain will shoot up next year with the release of Igby Goes Down, a film in which he plays opposite Jeff Goldblum and Susan Sarandon. It opened in the States this summer to good reviews, particularly Culkin's performance, in which, observed the New York Times, "not a false note sounded."

Culkin has floppy sable hair and is soft around the edges, as if a few stages shy of assuming his final outline. He is full of self-doubt, surprisingly at odds with the reputation of the Culkin family, which, at the zenith of its fame in the 1990s, was splashed repeatedly across the front pages as a symbol of dysfunctional celebrity. Kit, the father, was portrayed as a man pursuing a very modern line in child-abuse, force-feeding his six children into the Hollywood film-machine, while the kids inevitably came out of it looking like deranged brats. Kieran was eight when he appeared in Home Alone as Macaulay's younger brother, but while Mac went on to earn $8m in the sequel, Kieran remained pretty anonymous, playing the younger brother role in films such as Father of the Bride.

He hates to make a spectacle of himself. He is, he says, "pretty inhibited." He has never taken drugs ("I don't want the hassle of having to lie to my children") and doesn't have fake ID to get into American bars. He has never been properly drunk, although he likes good red wine. He is part puritanical teen and part world-weary adult, prematurely aged by the publicity kiln through which his family has been passed. In his spare time between rehearsals, he frequently takes himself off to Pizza Hut with a book.

In spite of his best efforts, of course, his childhood was ambushed by his father's ambitions for him and the huge spotlight trained on his brother. "Eughgh," he says, "I went to somewhere called the Professional Children's School. It was a private school adapted to kids with tough schedules. Some years I wouldn't work and I'd be there the entire year. Others I'd be gone for the entire year." Culkin didn't have a choice. "I always knew that, for me, work was more important than school. It was brought to all of us and then some of us decided not to do it any more and some of us decided to keep going. When I was 13, my father was making us do it. It was always his decision. Eventually I got to learn to like it." He refers to his dad as a "stage father from hell."

Did he ever rebel against the demands being made of him? In a silly voice he says, "Can't you see the halo above my head? I was a goddamn angel." Then, angrily, " I was a fucking angel." He casts his eyes downwards.

Until the money from Home Alone 2 came in, the family lived in a tiny apartment, eventually moving to take up several floors of a luxury block near Central Park. This was in the years before his parents split up and Macaulay "divorced" them both. "I didn't know we were dirt poor. Our apartment was skinny, a narrow, long apartment. I used to think it was a mansion. We'd run around like little naked, pet kids. You see pictures and there's always dirt on our faces, little nugget kids, mice and cockroaches. We found out afterwards that the whole apartment was on the slant, like when mom would bake a cake it would be slanted. That was all fun. Then we got some money and" - his voice grows sarcastic - "I guess that was FUN too."

Was the press intrusion horrible? "Yeah," he says wearily. "I mean only for a year it got really annoying, during the custody battle, I guess. We went to school right next door to our apartment building, and you couldn't even walk there. It was about 16 steps from home to school and I had to be escorted by my principal. And my brother didn't go for a couple of weeks. The school understood. They said, 'Don't go outside when it's like this'." Was he offered counselling on how to cope? Culkin is suddenly furious. "That's so wrong to do that, to counsel a kid, I don't know, maybe it's not wrong, but to sit down and stroke it and go, 'it's OK' and reassure him. No! If it's rough it's rough, that's what it is, you don't try to make it sugar-coated." He was 14 at the time. "I didn't care. It's like, 'This is me, not my brother'. I walked my 16 steps."

Did all the attention Macaulay got upset him? He shrugs. "I didn't care." A pause. "It was a little bit of pressure off me, I guess. I mean, I saw people mob him on the streets. I was always very thankful never to get mobbed." No jealousy? He laughs nastily. "Are you kidding? Trust me, if even the most star-hungry person in the world saw that, they would never want fame. It would get ridiculous, really physical, people grabbing his sleeve." He looks disgusted. Did he hate his father for it? "Well, I haven't spoken to him in five years." Are any of his siblings talking to him? "No. Nobody's talking to him. He's out of the picture." Is he curious about him? Icily, "No. I'm not curious."

The turning point for Culkin came when he was 16 and filmed The Mighty, with Sharon Stone, the first film he made free from his father's influence. Up until that point, he was adamant he would throw in acting the minute he came of age. But the film's British director, Peter Chelsom, encouraged him to stay with it. After making eight films, he says it was the first acting experience he actually enjoyed.

With Igby Goes Down, Culkin is starting to get noticed in his own right, and not in the crazy way his brother was. "I like the way I get recognised," he says, looking happier than he has all interview. "It's a very cool thing. In New York, I walked into a diner and this guy came up and said, 'Hi, buddy, I saw you in Igby, you were good'."

If it never gets more emphatic than that, will he be happy? Culkin beams. "That's the best way it should be. For someone to come up to you, say, 'Hey man, that was a cool film,' and to walk away."

· This Is Our Youth box office: 020-7494 5085