But now that his new play, Cleo, Camping, Emmanuelle and Dick, is about to open at the National, Johnson has gone all Garbo. He's not talking to anyone, I'm told, because he wants to let the play speak for itself. Which seems a bit unnecessary. He's never had reason to complain about his work being misinterpreted in the past, and having Cleo premiered at the National must feel like a personal vindication for the man whose 1993 play, Hysteria, the self-same company refused to commission.
Johnson has dismissed his work with a casual, "It's an irrevocable slide into light entertainment", but no one else does. His ability to blend true farce with contemporary drama has made him one of this country's leading comic playwrights. He uses the Ray Cooney trouser-dropping, bed- hopping routines as the structure for his writing, but underscores the text with psychological investigations into behaviour where anyone with the slightest pretensions is done for. The proximity of the comic to the sadistic makes the desperation of the lives laid bare more striking.
Cleo is Johnson's first play since his 1994 success, Dead Funny, and its genesis has been every bit as torturous as the time-scale might suggest: "In the past it was enough for my plays to be vaguely intelligent, but now I couldn't bear it if people didn't have a great time, too." He seems to have no grasp that his major talent has always been to entertain. The clutch of awards he's been showered with hasn't impinged much either; he probably reckons he just got lucky or that every other play just happened to be even worse than his.
It's almost as if Johnson finds the whole process of writing too unrewarding, and he frequently moans about winding up as sad and depressed as Arnold Wesker and Peter Nichols. He would much rather have been a stand-up comedian, but he lacks the ability to improvise. His scripts are often peppered with blanks and the word "Joke" while he struggles to come up with the killer gag. "When you fail to rise to the demands of playwriting, the best you can do is go down the shops or nip back to bed. It's a slow grind down to stasis and a slow crawl out of it. Being over 40 is a dangerous area for a playwright. You try to be more truthful, but what is there to be more truthful about? Falling in love with a younger woman ... that's about it."
Which just so happens to be pretty much what Cleo is all about. The play takes place over a period of 14 years from 1964 to 1978 and is set backstage during the making of four Carry On films - hence the title - and revolves around Sid James's infatuation with Barbara Windsor.
For someone who prefers to keep his personal life a closed book and who insists that he's astonished by people's obsession with the rich and famous, it's strange that Johnson has made his name by stuffing his plays full of well-known personalities. Insignificance featured Albert Einstein, Marilyn Monroe and Joe DiMaggio, Hysteria had Sigmund Freud and Salvador Dali, and the ghosts of Frankie Howerd and Benny Hill loomed in Dead Funny.
"I don't do it to promote celebrities as cults," Johnson says. "I do it as a quick way into talking about something. You don't have to explain who Einstein is. Ibsen wrote those hugely tedious first acts full of children running about so that you cared about the people who owned them. If he'd had my bright idea, he could have made his plays a lot shorter."
This is typical Johnson. A touch of humour to deflect an awkward question. For while his answer has a ring of authenticity, it is not wholly convincing. To use the same device four times suggests a fascination for celebs that goes beyond idle curiosity. A similar sort of ambivalence characterises his writing for women.
Over the years, Johnson has gained a reputation for being one of the few male playwrights who write sympathetic roles for women. Yet, for someone who has been credited with such insight into the female psyche, he appears genuinely confused by the way women behave. But rather than admitting this - or perhaps not even realising it - he opted for the safety of political correctness.
All this may be about to change, though. "For some time I've been pretending to understand women with some mysterious success," Johnson joked in February. "But I'm beginning to wonder if I haven't given them the benefit of too much doubt. It seems to me that both sexes always will have different agendas, and that we've just developed a set of games to pretend it is not so. In other words, women try to achieve male power by using their sexuality in a male way, and men try to crush women's aspirations to power by pretending to be women."
In other words, women are as duplicitous as men. He went on to warn that women had better watch out when he wrote another play. It'll be a few days yet before we know whether Johnson has done what he promised, but if I were Barbara Windsor I wouldn't be feeling too comfortable right now.
`Cleo, Camping, Emmanuelle and Dick' is in preview at the Lyttelton, National Theatre, London SE1 (0171-452 3000)