Adam Godley is one of Britain's finest actors. He is also one of Britain's quietest, with an extreme reluctance to talk about himself.

In a windowless South London room, those involved in the world premiere West End stage version of the multi Oscar-winning movie Rain Man are taking a break. First to the kettle is producer Nica Burns, who looks remarkably calm considering that her production has changed directors. David Grindley had to drop out for family reasons, so Terry Johnson has dropped in.

"Oh, that's old news," says Burns dismissively, even though the announcement was only made that morning. "I was much more nervous about the chemistry."

Burns is referring to her two, very different, leading men. One is Hollywood's Josh Hartnett - star of Pearl Harbour, Black Hawk Down and The Virgin Suicides. The other is Adam Godley, one of British theatre's finest - though not famous - actors.

Hartnett has been cast as the good-looking smooth operator Charlie Babbitt, played by Tom Cruise in the 1988 movie. Godley is Charlie's older brother Raymond, an autistic savant and the role that led to Hoffman's second Oscar. It is hard to think of two more contrasting actors. Hartnett is a Minnesota-born 29-year-old Hollywood heart-throb. Godley is a rather gawky 44-year-old Brit - born in Amersham, the son of a solicitor father and magistrate mother, and, despite a string of acclaimed stage performances, he has only recently been propelled to top billing.

He was nominated for an Olivier for his performance as Kenneth Williams in Terry Johnson's play Cleo, Camping, Emmanuelle and Dick (which Johnson also directed); he appeared with Alan Rickman in the West End and Broadway productions of Private Lives; he was the retarded Michal in Martin McDonagh's grim fairytale The Pillowman; he was a member of Mike Leigh's Jewish cast for the National's Two Thousand Years, in which he played the disillusioned Jonathan.

And while playing Jonathan, he also reluctantly stepped in at the last minute to take on his greatest acting challenge - that of the title role in Howard Brenton's searing New Testament play, Paul. Yet despite all this, there have been no major interviews with Godley. He is the quiet man of British theatre.

"It's always a risk," continues Burns. "You just never know until actors get together. But when I saw Adam and Josh in the rehearsal room, I practically went on my knees to give thanks."

In walks Hartnett, cool in his tracksuit and beanie hat, but still bearing the intensity of the rehearsal room, as does the grip of his handshake. As he waits for the kettle to reboil, I ask about that all-important chemistry. "I feel like we've got a real connection," he says of his co-star. "I'm pretty sure Adam feels the same way. We're playing brothers, so its important we have that."

Which is pretty much the answer you would expect. But then Hartnett adds, less guardedly: "At least, that's what I feel. If Adam doesn't feel it, he's a great enough actor to fake it."

In walks Godley, calm and quiet. He makes himself a peppermint tea. "I would hope I would never fake it," he says when, after the room has cleared, Hartnett's comment are brought up. "I haven't worked with Josh before. But fortunately we have discovered we work in the same way. We discovered that very quickly."
For Godley, work is all-important. He is happy to talk about the process, the role and the show - less so about himself. "I'm not really here to sell myself. I understand that people are often very interested about private lives, but I'm not a star, I'm not a celebrity and I just think I'm not that interesting." But these things can be hard to separate. Especially when actors have been specifically cast to draw on their personal lives, which is what happened when Leigh chose him for his first Jewish play, Two Thousand Years. Godley accepts the point, though only tentatively, as if answering it might open the floodgates.

"I grew up near Watford, a sort of suburban commuter-belt land. My family had lots of relatives in North London and Golders Green. My parents [now retired] were heavily involved in charity and the Jewish community."

Home, says Godley, was "semi-secular. But let's just say that I had enough of a feeling of a typical North London Jewish community, where Two Thousand Years was set, to be able to draw on it."
It was his first Jewish role. Then, if you count early Christians as Jewish, came the second. While Godley was playing Jonathan, Paul Rhys, who was to play his namesake in Howard Brenton's highly anticipated return to the National, fell ill and was unable to open as Paul. Ten days before the first night, director Howard Davies turned to Godley. After reading the role - a massive part that demands the actor to transmit the terrifying certainty of a man who has no doubts about his faith - Godley said no. "Then I made the mistake of having lunch with Howard Davies." Following that, Godley said yes.

"The first preview of Paul, stepping out on to that stage, was the most fear I have experienced. Walking out under-rehearsed and under-prepared - that's the clichéd actors' nightmare. You don't know who you're playing. There are thousands of people sitting, waiting for you to do something and you haven't a clue."
But when the reviews came out, Godley's performance was critically acclaimed. In fact, it elicited some of the best of his career. It was, though, an experience made no easier by the National's repertory season, which demanded he play Jonathan in Leigh's play on one night, and Paul in Brenton's play the next - both on the same stage. "I would think, wow, this time last night I was in that world and now tonight... but of course they were curiously linked in a strange, not so tenuous way. They were both Jewish."

And if his Jewish upbringing was the source of his performance as mild-mannered Jonathan in Two Thousand Years, what on earth could he draw upon for his portrayal of the fanatical Paul? "Waves that batter you when you're in a storm in your life," is Godley's oblique answer. "I've certainly had those. That's probably as far as I would go," he says, his guard back up.

It may be his short marriage to fellow actor Alex Belcourt that he is not talking about. If it is that, the storm is over. He now lives in Los Angeles, where he is "very happy in a relationship".

As the roles get bigger - his films include Charlie and the Chocolate Factory and the latest X-Files movie - it is going to get harder to separate his private and professional lives, however much he detests celebrity culture. "My life and career go its own route," he says philosophically. "I just try really hard to be led by really good work."

Rain Man is at the Apollo Theatre, London W1.
www.apollo-theatre.co.uk