As attempts to assert control over interviews go, things do not get off to an encouraging start with Mark Haddon. "So now we have this game of chess, in which you ask me what my new play is about, and I choose not to tell you what it's about," says the Whitbread Award-winning author, leaning back in his chair in an Oxford bistro and looking a bit too pleased with himself for my liking. "I suppose I ought to say something interesting instead. Or perhaps I should find out what you've gleaned from the press release. What do you think it's about?"
Well, er, according to the Donmar website, your play features a female character who sounds a bit schizophrenic, so, um, dissocia? "Ah, you are thinking of Anthony Neilson's  play The Wonderful World of Dissocia!" he says triumphantly. "I'll admit I was thinking quite a lot about that play. My play might be about bipolar disorder. But I'm keeping it close to my chest. I will tell you it has five and a half characters, though."
The Donmar Warehouse are also keeping the script for Haddon's play Polar Bears firmly under wraps, although this is now theatre policy for new Donmar plays, rather than a particular expression of paranoia on their part, or even Haddon's come to that. Still, trying to get more information out of Haddon is a bit like playing truth or dare with someone who is selectively deaf. "It's formally very weird," Haddon will admit, shaking his head. "Very weird. And I suppose people will see it as me ticking the disability box again. But it's funny when you start writing. You often don't know what might be hovering in the back of your mind."
Polar Bears is Haddon's first play. He is, of course, known most for The Curious Incident of the Dog in the Night-Time, his outstandingly sad and uplifting 2003 debut novel for adults which waltzed off with the Whitbread Book of the Year award and went on to sell more than two million copies. That was a perfectly formed novel about family breakdown narrated by a 15-year-old boy called Christopher who loves lists, numbers and accuracy and hates the colours yellow and brown, and being touched, and who had what was commonly assumed to be Asperger's. (Haddon is now thoroughly irritated that the word Asperger's appeared on subsequent editions of the novel, because now everyone imagines that he is an expert and he keeps getting phone calls asking him to appear at lectures.) He followed it up in 2006 with A Spot of Bother, a quietly intense, deeply bleak novel about a middle-aged man caught in paroxysms of despair over the fear he may be dying. The book's divergent reviews ranged from "enormously engaging and funny" to "undemanding" and "pedestrian". In between, he published a book of poetry (The Talking Horse and the Sad Girl and the Village under the Sea), wrote a TV drama called Coming Down the Mountain (about a boy with Down's Syndrome) and now he has turned to playwrighting. "It's been appallingly difficult. It's taken me seven years. And during that time I've written the most appalling piles of crap. I've basically been writing the opening of many crap plays over and over again."
By his own confession, Haddon is someone who likes to constantly change tack; he is also an accomplished artist. His website is testament to the magpie nature of his mind: it's subtitled "random notes about things that interest me" and in its hotchpotch arrangement of blog postings, illustrations, essays and diary entries gives as good an indicator as anything of the wired, restless nature of Haddon's creative thought processes. Still: poetry, novels, art, TV: now why theatre? "I always want to do something different," he says, perusing the menu with some frustration. "Life as a vegetarian is one long line of risottos," he adds cheerily, opting for spinach risotto with poached egg. "And I've wanted to write for the theatre for a long long time, really ever since I saw Tony Harrison's  play Trackers of Oxyrhynchus. All these nice middle- class theatre-goers: you could hear the fear and clenching in the room. Increasingly I've found theatre really interesting to watch. It took me a long time to come out as someone who doesn't like film. It's a bit like when people say they don't like books: you get that sharp intake of breath."
He is visibly pleased by Polar Bears, which stars Jodhi May in a production directed by Donmar associate director Jamie Lloyd, and despite not wanting to talk about the play, keeps drawing the conversation back to what he has learned as a fledgling playwright. Haddon, you sense, is a man with obsessive tendencies of his own: since embarking on this new career in the theatre, he seems to have seen nearly every new play that has premiered in London over the last few years, despite living with his two young sons in Oxford where his wife, Dr. Sos Eltis, teaches English at Brasenose College. He talks a lot about sitting in theatres with a notebook watching how the whole thing works – not so much the nuts and bolts of plot and character, but the more imperceptible alchemy whereby energy and structure work their ineffable magic on the audience. "I sit and think: how did they do that? For me, Curious was a book about reading and books, and how people fill in the gaps when they read something on the page. But everyone said it was about Asperger's. I suspect this is what will happen with Polar Bears too – people will say it's all about bipolar disorder. Actually, it's not. It's about theatre and what you can do on stage."
Still, for all his protestations, Haddon is undeniably interested in mental and physical health, although if that suggests some sort of campaigning dimension, in person his interest in the subject comes across as more forensic and detached. In the past he has put this fascination down to having worked for Mencap when he left university, even though that barely seems to explain a flurry of novels, screenplays and now a stage play about various mental and physical disabilities – Asperger's, chronic anxiety, Down's Syndrome and now, possibly, bipolar disorder. "I am really interested in eccentric minds," says this extremely ordinary-looking man. "It's rather like being fascinated by how cars work. It's really boring if your car works all the time. But as soon as something happens you get the bonnet up. If someone has an abnormal or dysfunctional state of mind, you get the bonnet up."
Yet this glib-sounding attitude belies the underlying charge of Haddon's novels. Christopher in The Curious Incident may not have access to a conventional emotional vocabulary, but when he gets upset, bewildered, scared or frustrated, he rolls up into a ball, and the effect instantly and brilliantly hardwires the reader into the off-kilter but wholly humane pathology of his particular world view. George, the protagonist of A Spot of Bother, is a bit like a modern day Mr Pooter, living in a mundane world whose daily surface co-ordinates include cups of tea and building a new garden shed. Yet he is driven so mad and scared by what he believes to be a malignant tumour on his hip he attacks it with a pair of scissors, in a standout chapter that is as darkly comic as it is deeply distressing. Both novels are highly compassionate studies of people whose all- consuming emotional afflictions put them at a terrifying distance from the everyday world; Haddon's great skill as a writer is to find a way of reaching out to their and our common humanity. "People in situations such as Christopher's and George's really aren't often that abnormal," he points out. "People say to me: 'how did you get to grips with Christopher's mind? It's amazing'. I have to point out that the only reason people think they know him is because all the bits of him they are drawn to they probably share." He stops and ponders for a minute. "Psychiatric illness and learning difficulties are also great subjects for black humour. That whole territory puts readers on the back foot and makes them a little bit nervous. What's left? I suppose depression. That's a bit boring though, isn't it?"
"Well I can't think of any good fictional accounts of depression," he says, that glib tone creeping back again. "One of the problems is that depressed people often don't do very much. You just lie on the sofa for six months eating chocolate and watching DVDs. There's probably a certain kind of experimental novel in there, but I doubt there's a play. But lately I've realised that really I'm interested in families. Stories about mental aberration and oddity only make sense in context. Just how do people live with someone who is peculiar, gifted, strange or alien? It's odd because there's a little part of me that wants to write about exotic, strange bizarre subjects. Instead I've rather reluctantly realised that what I write about is families."
Has he ever felt depressed? "I suffer depression only in the sense that I am a writer," he says. "We don't have proper jobs to go to. We are on our own all day. Show me a writer who doesn't get depressed: who has a completely stable mood. They'd be a garage mechanic or something. Anyway, I don't think you become a writer unless you are aware of the workings of your mind."
Haddon didn't grow up wanting to be a writer. Instead he wanted to be a mathematician. "That was a ridiculous idea," he says now. "You have to be brilliant to be a mathematician, or not at all." Still, his interest in the sciences is not that surprising given that he seems to view human behaviour rather like one enormous maths puzzle; Christopher in Curious also uses maths as a way of understanding the world. Haddon boarded at Uppingham School, which he appears reluctant to discuss ("no one is happy at boarding school are they?") It wasn't until he was older that he started reading, and not until after he left Oxford University (where he studied English) that he started writing. "I seem to have developed an allergy to maths now that will not go away," he says. "Although you are right: I can see the appeal of the order that maths offers. I just find the art of writing or painting much more interesting; it's messier and harder and you never do get the answer, you just get things that feel like the answer for a while."
Winning the Whitbread may have made him comfortably well-off, but it hasn't necessarily made him feel secure. "I kind of forget about the Whitbread," he says. "You don't wanna think about that stuff otherwise you start thinking you're really good. It's the beginning of the long slide. I'm sure it's the same with anyone who's had big sales or prizes – it's in the past, isn't it? You still get out of bed the next morning thinking, 'what shall I do now?'" But he has got his debut play on at the Donmar, which is no mean feat: surely he can relax a little bit? "The problem is I spend years thinking that I desperately want success to happen; and as soon as it happens, I think, 'oh, this is something I can do, so just how impressive can it be?' Basically I have singularly failed to be impressed by myself."
He copes with writers block and the eye-swivelling consequences of a manic need to edit by running or by painting. "After about an hour and a half to two hours running, your brain gets quite small, and that's quite good," he says. "I think of myself as lying around doing nothing most of the time, and then I look back and think: 'I seem to have produced rather a lot doing that'. But I do need to be locked into a room with no guitar or washing up in the sink with a timer on the lock to really knuckle down. Part of the joy of travelling down to London recently for rehearsals was the amount of time I could spend writing on the train and at a café at Paddington station."
Haddon gives the impression of being compulsive, solitary, slippery, needy and, for all his seeming ordinariness, frankly a tad eccentric, placed slightly at awkward right-angles to everyone else. "I used to have this fantasy that I would belong at some point, as though there were some secret club somewhere," he agrees. "But as everyone points out, there is no secret club."
'Polar Bears', Donmar Warehouse, London WC2 (0844 871 7624; Donmarwarehouse.com) previews from 1 April, opens 6 April and runs to 22 May
Top Marks: Selected Haddon highlights
Gilbert's Gobstopper (1987)
Follow the adventures of Gilbert's amazing gobstopper as it travels from his mouth to the bottom of the sea and to the wilds of outer space. Although not a major seller, critic Carolyn Polese commented in a 'School Library Journal' review that Haddon's "irreverent entertainment will tickle many a funny bone."
The Sea of Tranquility (1996)
An inspiring story about the first moon landing, depicted through the eyes of a child, with beautiful pastel illustrations. One reader stated that "if you're over 40 you'll have a tear at the end of the story; thanks to the writer, not the illustrator."
The Curious Incident of the Dog in the Night-Time (2003)
This book has been hugely successful and has won a string of prestigious awards, including the 2003 Whitbread Book of the Year. It follows the story of a 15-year-old autistic boy, Christopher Boone. After discovering that his neighbours' dog has been killed, Christopher sets out to solve the mystery, which in turn slowly upsets his carefully constructed world.
The Talking Horse and the Sad Girl and the Village under the Sea (2006)
Haddon's first book of poetry finds the author harnessing all his powers of human feeling and black humour. The collection met with a divergent range of reactions on its publication; while some saw it as "unusually funny, frank and wry", others found the book a "waste of money".
A Spot of Bother (2006)
Haddon's second novel for adults was shortlisted for the 2006 Costa Novel Award. Whilst George is convinced he has cancer, his wife is having an affair with George's former colleague, their daughter Katie is worried she is marrying for the wrong reasons and their son is having boyfriend issues. One critic called it, "a curiously good follow-up". Rosie Barcroft