THEATRE / A star for theatre's New Age: Is Mark Rylance the finest actor of his generation? His supporters are diverse: a Broadmoor inmate, Alan Bennett and now Paul Taylor

The most flattering review Mark Rylance can ever have received came not on Broadway but in Broadmoor, and from someone who could claim a certain expertise. The top-security hospital was playing host to the RSC, who had just mounted a one-off performance of Ron Daniels's 1989 production of Hamlet. This was the version in which Rylance, famously clad in filthy striped pyjamas during the 'antic disposition' scenes, took the role of the Prince by what you'd have to call brainstorm. After the actor had taken his bows, one of the inmates rushed up to him and said, in hats- off tones, 'You were really mad - take it from me, I should know, I'm a loony.'

For some of the more conservative critics, the dangerous, electric levity of Rylance's derangement - his 'mooning' at courtiers who crossed him, his sick-joke emphases (like the jaunty 'goodnight' flung at his mother while lugging off the corpse of Polonius) and the self-hating spurts of violence (spitting in Ophelia's face in order to rub off her make-up) - made this a Hamlet the Bane. What amazed many of us, though, was the way this bonkers, alternative comedian of a Prince also managed - thanks to Rylance's haunted sweetness of countenance and that intense but wary rapport he can build up with an audience - to project Hamlet's infinite solitariness and injured spirituality better than any contender within memory.

The idea that, in Rylance, we have the finest actor of his generation began to take shape around that time. Next week, in a production that begins at the West Yorkshire Playhouse and then transfers to the Donmar in London, the 34- year-old veteran takes on a new challenge. Swapping parts on different nights with the American actor Michael Rudko, Rylance will get to play both the brothers (the dapper suburban screenwriter and the lumbering, slovenly drifter) who wind up in hapless role-reversal in Sam Shepard's True West. This seems a good moment, then, to try to take stock of his talent and pinpoint the elements that give his acting its peculiar power.

Alan Bennett's diaries contain this allusion to him: '27 May 1986. Roger Lloyd Pack is to play Kafka in Kafka's Dick. We also see Mark Rylance, an actor whom I don't know and who is also very good. Totally self-absorbed to the point of eccentricity, he's the first actor we've read who makes sense of Kafka's desire to be somebody and nobody at the same time . . . Rylance has a lovely, appealing face and marvellous directness.'

The revealing detail there is Bennett's reference to the way the actor managed to get right inside Kafka's extreme ambivalence about whether he wanted to join the literary pantheon or not. For, when you look back at the roles in which Rylance has made an indelible impact, here and in America, it's striking how many are portraits of emotional outsiders who are either deeply ambivalent about their desire to belong or incapable of achieving real contact.

Who could forget the moment in the RSC's Peter Pan (1983) when Wendy read a bedtime story to the Lost Boys, and Rylance's Peter - like some vulnerable but mysteriously unreachable incipient teenager - affected indifference and sullenly carved himself a Pan pipe. But his Peter, as one reviewer finely put it, 'nearly lost his emotional virginity (here), so moved was he'. Rylance's achievement was to make the yearning and the defensive, sprite-like heartlessness feel like parts of a seamless whole.

Nearly 10 years later, his award- winning performance as John Healy in Screen Two's 'The Grass Arena' brilliantly embodied the paradox of childlike optimistic openness and hooded, old-before- his-time despondency that met in a rejected young man who, wherever he turned looking for companionship, found only brutal competitors. Slight of stature, Rylance, who'd be the perfect model if you wanted to sculpt a mythological faun, also has the capacity to hunch himself into a Barry McGuigan-style, aged-kid look which was perfect when playing Healy, for whom stress was a major muscle-knotting experience.

Born in Kent, Rylance was brought up in Milwaukee by English-teacher parents. The cut-off introspectiveness that is part of the impression he makes may date from the pronunciation problems he had as a child. Until he was seven, no one could understand what he was saying except his brother. Before coming over to study at Rada, he had already played Hamlet as a 16-year-old at school, a role that, to someone who liked to go into 'imaginative mazes', came, he says, like 'suddenly hitting heroin' after fooling around with soft drugs.

He is famous (or, perhaps, notorious) for his attempted application of hermetic teaching to Shakespearean drama, his dressing room when he played Benedick in the West End boasting a diagram entitled 'The Cabbalistic Tree of Life in Relation to Much Ado About Nothing'. Indeed, he nearly lost the flat he'd mortgaged when the rainswept summer of 1991 poured a steady stream of cold water on his Phoebus Cart outdoor- production of The Tempest, an alchemical interpretation of the text which toured to leyline sites like the Rollright Stones in Oxfordshire and which described itself, somewhat ambitiously, as 'the acupuncture needle for Britain', intent on restoring broken lines of energy to the body of the earth.

Talk to him of his more recent off-Broadway performance of Henry V and he'll speak in terms of 'the karma of the nation' and 'Orphic descents', rather than of the brute political realities. But to meet him is to be convinced that he's more Blakean visionary than irritating crank. As Thelma Holt, who produced him in Much Ado, says of the Rylance mind, 'He'll always be in his own jungle, but it's not wild and he does walk around in it. This is no battery hen; he's free-range.' Like Matthew Warchus, who directed that production and who has teamed up with Rylance again for True West, she admires the instinctive radicalism with which he re-invents roles and working methods. He reminds Warchus of the Tom Hanks character in Big, the child in a man's body who walks into a toy store and can see, as the adults can't, how the toys don't work.

His brilliant re-invention of Benedick as a pig-headed, hang- dog, droopy-moustached Ulsterman was a completely original take on the character. Benedick's apartness from his fellow-soldiers, his wariness about committing himself emotionally and that occluded insider-outsider status that is Rylance's speciality came over all the more hilariously from this touchy, humourless curmudgeon.

'The whole must be peopled': Rylance's Benedick, succumbing to the prospect of romance, delivered that line with all the poetic uplift of someone reluctantly accepting a timeshare in Arcadia on the drab, duty-calls basis that 'cows must be milked'. New- minting the dialogue in this way, he proved himself a dazzling master of comic timing, a gift which will be on display in more ways than one and in more roles than one in True West.

'True West' is at the West Yorkshire Playhouse (0532 442111) from 7 Oct to 5 Nov; Donmar, WC2, (071-369 1732) from 9 Nov to 3 Dec

(Photographs omitted)