If Anthony Sher could travel back in time and witness one theatrical performance in history, it would have to be to the Drury Lane Theatre, in 1814, when the great Shakespearean actor Edmund Kean was appearing as Richard III.

His diminutive stature reminding some critics of Napoleon, Kean startled an audience accustomed to the heroic strut and declamatory style of the day's tragedians by bustling across the stage "as if completely unaware of them" and speaking the famous opening soliloquy in natural tones.

"By Jove - he is a soul!" wrote Lord Byron. "Richard is a man, and Kean is Richard!"

You'd think, some 400 years down the line, there can be no more Shakespearean conventions left to break. But in 1985, in an Olivier Award-winning performance for the RSC that has itself become legendary, Sher also reinvented Richard. Inspired by Queen Elizabeth's description of him as "that bottled spider", Sher played the king on crutches, lurching about the stage like a trapped insect and suggesting a man made "pathetic and vulnerable" by physical deformity.

"That's the absolutely astonishing thing about Shakespeare and an explanation for his genius," Sher tells me. "His material is so rich that it lends itself to almost endless interpretation.

I hope, in 400 years, we'll still be interpreting his plays freshly."

Now Sher is starring in a revival of a play all about the life of Edmund Kean, and it even begins with Kean delivering the opening speech from Richard III.

"You could get very daunted by the idea of, How do I act Kean?'" says Sher. "Well, you can't. You can only try and do it as well as you can. We've borrowed aspects of my RSC performance, but the look is based on paintings of Kean in the role. I suppose it's kind of a hybrid of Kean and me".

Now that should be a performance worth seeing.

Kean, a man of whom Coleridge declared, "Seeing him act is like reading Shakespeare by flashes of lightning", played Brighton's Theatre Royal numerous times during his career and is frequently held to be the greatest performer of the 19th century.

His genius was apparently instinctive. Born in poverty and abandoned by his mother not long after his birth, by the age of ten he had performed Richard III in its entirety, taking all the parts and using his aunt's curtains for Richard's campaign tent. By 14 he was playing Hamlet and was summoned to perform at Windsor Castle by George III.

He made his Drury Lane debut in January 1814 in The Merchant Of Venice, dispensing with the false nose and ginger wig of convention to render Shylock, hitherto seen as a comic villain, as a complex, tragic figure.

Doing away with the declamatory "traffic-signalling" style of his rival John Philip Kemble, Kean became the father of naturalistic acting and mesmerised London audiences with his portrayals of Shakespeare's great tragic roles for nearly two decades.

And yet, early in his career, the celebrated actress Sarah Siddons proclaimed, "there is too little of Kean to make a great actor".

"I've always been fascinated by Kean," says Sher, who has played many of the same roles in his time at the RSC, "because in the great line of British actors - which roughly goes Burbidge, Garrick, Kean, Irving, Gielgud, Olivier - he breaks the mould completely. Leading actors were supposed to be tall, handsome and noble. Then suddenly there is this short, dark, rather demonic creature.

"Being short and dark myself - and possibly rather demonic, people might say - he made me feel I might be able to be taken seriously as a leading actor."

Born in Cape Town to a Lithuanian Jewish family, Sher was pursuing acting "with no great clarity" when he moved to England at the age of 19 and realised that "Olivier, Gielgud and Redgrave were all a shape that I wasn't".

As a child he had been "very withdrawn and introverted", spending hours alone in his room painting and drawing. "So my parents sent me to what were called, in those days, elocution classes, but were actually little acting classes. I found it very liberating that by becoming other people I could overcome my own shyness."

The young actor got his break when, in 1981, he was cast as lusty left-wing sociology lecturer Howard Kirk in the hit BBC2 series The History Man. This led to him going to the RSC, where the director Terry Hands took a particular interest in shaping him as a classical actor.

Sher has since led numerous major productions, including Tamburlaine, Stanley and Macbeth, as well as starring in the films Mrs Brown and Shakespeare in Love. Knighted in 2000, he has been called the greatest classical actor of his generation. But he has continued to suffer from shyness and insecurity, leading to a cocaine addiction for which he was treated in 1996.

"In Kean's case, I think it was more a battle to build himself up," he says, when I ask further about his affinity with the historic tragedian. "He was illegitimate, suffered extreme poverty. For him it was often a real battle just to survive."

Kean lost the battle, aged 46, after a life-long struggle with alcohol.

Touring regionally prior to a West End transfer, Kean, the play, is fascinated by the contradictions in the great actor's life. Penned as a romantic drama by Alexandre Dumas just three years after Kean's death, it was rewritten by Jean-Paul Sartre in 1953 and is now directed by Adrian Noble in a translation by Frank Hauser.

Sartre's Kean is a rather monstrous figure, who has turned his entire life into a theatrical performance full of egotistical soliloquies, mammoth drinking sessions and - despite his marriage to the actress Mary Chambers - passionate love affairs conducted via a trapdoor in his dressing room, which admits female admirers.

It is currently being sold as "a stunning behind the scenes portrait of theatre life, perfectly capturing its addictive appeal", although the theatre critic Michael Billington, reviewing a Nineties production at the Old Vic starring Derek Jacobi, called Sartre's script "vulgar claptrap".

"I've seen it a few times and it fascinates me as a play," says Sher, who had discussed the project for a number of years with producer Thelma Holt. "Sartre, of course, layers Dumas' romantic adventure with philosophy, the actor asking himself, What is reality, what is theatre?' I've always been interested in the fact that, well, acting is a very, very peculiar job to do. There are such perceptive speeches about that here that you can't believe Sartre wasn't an actor.

"He also does the most amazing things with breaking down the fourth wall, so that there are times when we talk directly to the audience and break the make-believe. It makes for a very strange - in the best sense of the word - and, I think, rather dangerous evening."

For Sher, another welcome and dangerous aspect of Sartre's script is his highly uncompromising portrayal of Kean.

"He's an alcoholic, he's a womaniser, he's a wild boy. He's a man hell-bent on destroying himself. I suppose in contemporary terms it's Oliver Reed or Richard Burton, one of those people whose life was on a kind of knife-edge between creative work and self-destruction.

"I never got, thank goodness, to the point where my cocaine addiction was destroying my work. I wasn't behaving as erratically or unreliably as we see Kean in this play. But I certainly know about the early stages of that.

"Personally I find it quite liberating, having been through a period of self-destruction, to now play that in the relatively safe environment of a play, on a stage, which begins at 7.45 and is over by 10."

Just as he seems never to have lost his natural shyness, Sher retains his childhood passion for painting and drawing. Earlier this year he held his first exhibition in ten years at the London Jewish Cultural Centre, to great success.

His speciality is portraits, in particular the theatrical portrait, which has become something of a dying art since the advent of photography. Sher has painted everyone from Ian McKellen to John Malkovich, and the exhibition also featured a number of self-portraits: Sher as Iago, the Fool in King Lear, Richard III "The human face is what fascinates me," he says, "and I think there's an overlap in acting there. Because I've got the eye of someone who paints, I've always enjoyed working with the designer on putting together the whole look of a character. Of course Kean is a slightly unusual case, because so often here he is getting ready to look like someone else."

The recent exhibition of Sher's work also featured a piece entitled The Male Line, which reportedly featured his father's ashes and traces of cocaine. In Sartre's play, Kean's private life becomes a public performance. I wonder how firmly Sher enforces the distinction between work and private life?

"What happened to Kean was that he was just hugely famous in a way similar to pop stars or sports players nowadays," he says. "That was basically a celebratory thing, until he had an affair and the tide of public opinion changed.

"In my own life, it has almost been the opposite thing of me making public aspects of my life. The coming out aspect, not only as a gay man but as a Jew - all these aspects of my identity that I hid at first when I came to this country - this has been a very important part of my life, really: to finally be who I am."

  • Starts 7.45pm, Thurs and Sat mats 2.30pm. Tickets cost £16-£27. Call 08700 606650