Anthony Neilson is never less happy than when he’s successful. His current play, The Wonderful World of Dissocia, won Best Play, Director (Neilson himself) and Actor at the Scottish Theatre Awards. It has now been selected by Dominic Cooke to launch his artistic directorship at the Royal Court in London. But you won’t catch Neilson cosying into the Establishment’s embrace. “The unanimity of enthusiasm for Dissocia slightly worries me,” he says. This is a writer to whom, according to fellow dramatist Mark Ravenhill. "the artist as outsider is a powerful myth".

For the first decade of his career, Neilson was the myth incarnate. He pioneered the so-called “in-yer-face” theatre movement of the 1990s; Ravenhill and Sarah Kane were his protégés. His breakthrough play, Penetrator (1993), defined the decade’s visceral, blood-and-sperm theatrical mode. In 1997 he had a porn star defecate on the West End stage in The Censor. His 2002 drama Stitching involved Auschwitz, sexual role-play and a woman who sews up her vagina.

These were plays that penetrated the grimiest recesses of our collective psyche to retrieve unforgettable images of tenderness and pain. We’re not talking Alan Ayckbourn here. And yet in recent years, Neilson has put fringe theatre, and enfant terrible subject matter, behind him. First he wrote a farce, of all things, for the Royal Court. Then Dissocia, and its companion piece Realism, were given illustrious main-stage slots at the Edinburgh International Festival.

But Dissocia’s London transfer can hardly be seen as a consummation of Neilson’s love affair with the mainstream. After all, he has waited a frustrating two years for it to happen. (Rumour has it that a proposed 2005 Royal Court run was gazumped — ironically, given Neilson’s reputation — by Tim Fountain’s promiscuity confessional Sex Addict). As far as Neilson is concerned, he’s been banished from London since the failure of that 2002 farce The Lying Kind.He blames the Court’s former supremo, Ian Rickson, who betrayed “the need for theatre to support people when the risks they take don’t work out”.

And whatever you say about Neilson, he takes risks. He enters rehearsals with a blank page and writes in collaboration with actors. He refuses to work according to “the old model whereby you write a draft and go away and rewrite it and

A restless inquirer into theatrical form, Neilson is on the verge, he thinks, of defining nothing less than “the next movement in British theatre”. He’s calling it “psycho-absurdism” — a wild, nonnaturalistic theatre (akin, he admits, to the films of David Lynch) in which anything can happen. “I’m giving as much time to the inner life as the outer life,” he says. “When you dramatise mental states, your limits are only what the mind can think of. You can have humour, and surprising shifts in tone. You can have songs. You can have dance.” He thinks that it is the responsibility of artists to reach out to the “95 per cent of the population who think theatre is an irrelevance. My shows are very accessible. I’ve never believed that experimentation means unpopularity.”

The curse of theatre, says Neilson, is that many people see it as both educational and effeminate — “the two worst brushes you can tar anything with. Whereas what you want to see is something that moves you, that provides spectacle but also food for thought.”

Which brings us to Dissocia, a piece that stages the inner life of someone with dissociative disorder. Neilson’s heroine journeys to a Technicolor Alice in Wonderland-style realm ruled by the Black Dog King, where rapist goats stalk the land and borders are patrolled by insecurity guards. The play stemmed from Neilson’s own secondhand experiences of mental illness (about which he maintains a diplomatic silence), and asks: “Why is it that people who are mentally ill find it so difficult to take medication to normalise their behaviour?”

Not that he’s glamorising mental illness. “Critics who believe that a play must always be a veiled argument about something,” says Neilson, wrinkling his nose, “have interpreted Dissocia as weighing up psychiatric illness and treatment, as a ‘which one is better?’ debate, which would be extremely crass. What I dread is appearing to say: ‘Let the crazy people be free, man’.” To that end, he has “hardened up” the play’s uproarious, freewheeling first act, which he worried audiences found too easy to enjoy.

And that’s why we should take Neilson’s mainstream success with a pinch of rat poison. He talks of being populist — but he’s as much crowd unsettler as crowd pleaser. “I might have my sense of populist entirely wrong,” he admits. “Looking at my bank balance, there is a good chance of that.” But, however popular it was, those changes to Dissocia just had to be made. Because watching it, he says, coining what might be his own epitaph, “you should always feel that there is something just a little bit wrong”.

The Wonderful World of Dissocia, Royal Court, Sloane Square, London SW1 (www.royalcourttheatre.com 020-7565 5000), until Apr 21

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