For most of his twenties, Jason Priestley was someone else altogether. For great chunks of his working week he was Brandon Walsh, clean-cut all-American boy next door in the long-running soap, Beverly Hills 90210. He quit the series two years ago, aged 28, but as often happens with soap actors, quitting the character will take a lot longer - not least because Channel 5 is re-running them on Sunday mornings.

"One of the reasons I became an actor in the first place was that I felt more comfortable being someone else than being myself. Doing a show like Beverly Hills 90210 was a way of avoiding facing up to the problems of growing up. Whether or not you learn from your mistakes is what it is to grow up and become an adult. I feel a lot more comfortable in my own skin since I left the show."

It wasn't hard for Priestley to pretend to be a high school boy for eight years. Now 30, he remains boyish, vertically challenged and full of bounce. "As you can see" he says at one point during the interview, "I don't suffer from a lack of energy."

Brought up in Vancouver, he made his first screen appearance aged eight, but he insists he wasn't pushed into acting by his parents, even though his mother had been a ballet dancer in her youth. "I had a normal childhood. I didn't grow up in a Norman Rockwell painting … my family is as disjointed as anybody else's, but I was mostly happy."

The subject of childhood is relevant here because the Tony award-winning play in which he makes his West End debut this week, Side Man, is about a young man trying to make sense of his unconventional upbringing. "I play Clifford, the son of a veteran jazz musician father, and an alcoholic mother. He has taken care of his parents since he was a child and now, aged 30, he is leaving them to their own devices. He's damaged but he doesn't use it as an excuse for behaving badly. He's just trying to work things out for himself."

Indeed, Clifford understands all too well the ties that bind them to their jazz-loving cronies. "For all their joking and fucking up, they have a profound connection to their music, one they can only share with each other," he confides to the audience.

It is a particular challenge for Jason Priestley after so many years of pre-recorded acting, since he is not only playing to a live audience every night, but as the narrator of the piece, talking directly to them while the action on stage freezes. "Sure it's a heavy load because I'm on stage more or less the whole time. But it's what I was trained to do - I started out in theatre - and the other actors have made me feel really welcome. Some of them have been with the play for four years so they're very comfortable with it. I know I'll be a basket case on press night but I love the fact that I'm doing it in London. Audiences here are really intelligent."

The last time he did any theatre was ten years ago in Vancouver. "It was some weird, avant garde thing, with lots of shouting and power tools. It put me off doing theatre for a while."

So how come he stayed so long with Beverly Hills 90210? "I took advantage of the time I had there, learning about what goes on behind the camera. I had a great teacher in Aaron Spelling, the producer (Dynasty, Charlie's Angels etc). I was always interested in being a bigger part of the whole creative process, and Aaron gave me the opportunity. By the time I left, I'd directed 22 episodes and produced a few as well." This gave him the confidence to direct the documentary, Barenaked America, about the Canadian band, Barenaked Ladies, on tour, which should be opening here this summer. He has also directed a TV movie, Kiss Tomorrow Goodbye, which will no doubt surface on our screens in the not too distant future.

As an actor, he'll be seen next month in the psychological thriller, Eye of the Beholder, with Ewan McGregor and Ashley Judd, and an independent film, The Highwayman, starring Louis Gosset Jnr. Though he is friendly enough on the surface, living in the goldfish bowl of Los Angeles for a decade has clearly given Jason Priestley a deep suspicion of prying hacks, especially in the light of recent events in his private life. Not only did his marriage to a make-up girl break up after just ten months, but it turns out that he is in London on $50,000 bail, with a charge of drunken driving pending in the States. So much for the actor's claim that he lives "a frightfully normal life … going to the gym and taking my clothes to the dry cleaners."

Even as he says it there is a twinkle in his eye. Or perhaps it's a nervous tic. Either way, I doubt very much if Jason Priestley's three-month sojourn in the capital will go completely unnoticed by the tabloids.