Bob Hoskins is playing his best role to date. Mr Disingenuity. "I'm 53. I'm a short, fat, middle-aged man with a bald head. That's my image."

My question was whether his new West End role as a Viennese singing teacher in Jon Marans's Pulitzer-nominated Old Wicked Songs had rather more cultivated associations than did the practically indelible image that most people have of him. His first answer was that he had absolutely no idea what his image was at all. This puts him in a tiny group, as he must be the only person in Britain who doesn't. Then, when pushed, he proffered that quotable thumbnail sketch: the slightly laddish, self-deprecating Central Casting East Ender, and typically Hoskinesque.

He's good at quotable nuggets ­ "I have no ambition at all. I just go from job to job" ­ but it's harder to get him to commit to any kind of analysis of his work or himself. He clearly finds it difficult to take the advice that he has been dishing out to the rest of us on behalf of BT for so long. He does not find it "good to talk". He finds it rather aggravating.

Granted, he is in a lunch break from rehearsals at a crucial period in the pre-production process of a new play and Old Wicked Songs will be his first stage role for 13 years, but Hoskins appears cagey, and filters his responses to my questions through a palpable reluctance about doing interviews and a tuna sandwich.

It is easy to forget that Hoskins is an actor of considerable brilliance. His performances in Mona Lisa (which won him an Oscar nomination) and in the earlier The Long Good Friday stand high; but his subsequent movies have not stacked up much evidence of quality. Just pick three from the 15 he has made since: Super Mario Bros (1993); an enjoyable but pretty workaday cameo of Edgar J Hoover in Nixon (1995); and co-starring with Cher in Mermaids (1990) ­ all have been typical of the kind of work that pleased his bank manager a great deal more than it did audiences or critics. But up to the mid-1980s Hoskins saw glory days as a stage and television actor.

Success at the Royal Court in the 1970s with Bill Gaskill lead to the RSC and then to big roles at the National. Critics pronounced his Bosola, opposite Helen Mirren's Duchess of Malfi, definitive. His Nathan Detroit in Richard Eyre's original National Theatre Guys and Dolls and his Arthur Parker in Dennis Potter's Pennies from Heaven were hallmarks not just of his versatility but of his energy. John Schlesinger says now of his 1981 production of Sam Shepherd's True West, in which Hoskins co-starred with Antony Sher, that "he had the feel of a coiled spring. He was marvellous". And the reviews were stunning.

However, Hoskins's recent appearances on both small and large screens have made him a personality. You may not know much about his wife, his cat or whether he drinks a lot, but you think you know who Hoskins is. And that may be a bit of a curse for an actor. As Schlesinger says: "You can't completely subvert a personality"; and as Richard Eyre adds, rather more pungently: "Bob does light up a room. But when he wants to turn down the gas or lower the lights a bit, you almost feel cheated. It's like 'Bob Hoskins' has left the room." And turning down the lights and lowering the gas is exactly what his new West End role demands.

Old Wicked Songs is a two-hander. A young American piano prodigy called Stephen Hoffman, who has not been able to summon up the courage to play for almost a year, comes to Vienna to rekindle his genius at the feet of a renowned vocal accompanist. Instead, however, the maestro sends him to study with a cranky, failed singing teacher, Josef Mashkan. Their relationship is a series of often vituperative and hostile emotional minuets, shot through with both characters' pride and prejudices, and is set against the background of the Austrian presidential elections in 1986, which Kurt Waldheim, despite evidence emerging of his Nazi past, eventually won. It is a Vienna riddled with anti-semitism. Melancholic music by Schumann is woven in and out of the play, partly through the device of teaching it to the young pupil, and partly by using the songs to top and tail each scene. Why, after so long and so many other offers to return to the stage, did Hoskins choose this play?

"I've never really been offered anything that interested me before." Why did it interest you? "He's a very interesting character." I ask a number of times why this singing teacher is so interesting, and receive a number of not very revealing answers, such as "He's Viennese" and "I like a bit of a challenge". Finally Hoskins says: "These two outcasts are both failures. Mashkan is a complete washout. Dealing with Schumann's song cycle is basically about manic depression. And that's what he is, a manic depressive."

This is not to say that the play is depressing. It cleverly explores the clashes of the two cultures, American and Viennese, and the clash between the two men, one at the beginning of his life, with all the significance of history to learn, and the other an old man at the far end of an adulthood of failure, with only too clear a grasp of how damaged he has become.

"Mashkan is this extremely emotional . . ." Hoskins pauses. He seems about to be deflected away from explanation and into soundbite, as he already has been several times. But he continues: ". . . He is an emotional cripple. Because of this he does have a passion, he does have feelings and in this battle between them he's trying to force that passion into the boy. And the boy does start to feel and start to think. The two of them grow together. It's very much a two-handed play."

The other hand is James Callis. He is just out of drama school. By an accident that infuriated the director when he found out, I managed to sit in and watch an hour of the two of them rehearsing together. It was illuminating. Callis was trying to make something work. Hoskins was trying to help him. The easy conclusion was that in life, as in the play, Hoskins was the teacher, the young man the pupil. Wrong.

"Unfortunately, you came on a morning when we were trying to pinpoint something about James's character, Stephen," Hoskins said afterwards. "James has done the same thing with me. If it's a star vehicle, this play isn't going to work. He's got to be able to push me around as much as I push him. Blow the fact that I'm older than him."

In fact, watching Hoskins rehearse reveals an actor of great instinct. Quiet, unladdish, concentrated, he convinces easily that, as Richard Eyre says, "he has invented the 'Bob Hoskins' persona and it is a carapace". And this part is clearly stripping some of it away. Hoskins says: "Usually with me I'm not method. I don't live with the characters and I don't take them home with me. But this seems to be developing sides of myself which are becoming dormant." Like what? "Depression, fear."

Maybe at this point Hoskins is opening up? This play has obviously chimed with a moment in his life. But how is it affecting him? "I'm getting some very strange dreams, I must say." Such as? "What, you want to know my dreams?" Yes. "Well, f*** off." But you brought it up. "Well, I . . . well, yeah . . . it's a play you just can't do without it affecting you."

Hoskins gives the impression of a man returning to what he does best, which is opening a tiny gap in the shells of damaged men and showing us their emotions, as he did in Mona Lisa, Pennies from Heaven and The Long Good Friday. It may be that he is at a point in his life where, not just in his roles, he no longer needs the cheeky chappie that he has been for so long to protect him. And the benefit of that to us is that we will get back Hoskins the actor.