Although the Harry Potter books are children's books, like a lot of other adults I loved reading them. I got really excited by the stories. For me, they work well as mysteries but they're also very funny. You make Harry Potter's world so convincing. Somehow, it has a lot of depth.
It's nice to hear you say that because I spent so long constructing this world - five years writing about Harry before anyone read a word. It's embarrassing how many trees have died for me. I write everything down ... and lose it promptly. It's all stuffed into boxes.

I loathe books that have inconsistencies and leave questions unanswered. Loopholes bug the hell out of me. I hate getting to the end of a book and thinking, but if so and so had told Mr Y back in chapter three, it need never have happened. And so I try to be meticulous and make sure that everything operates according to laws, however odd, so that everyone understands exactly how and why.

I have a visual imagination. I know this isn't the case with all writers - some hear words as opposed to seeing pictures. But I see things, and then try to describe as faithfully as I can what I'm seeing. I have to imagine something clearly first, and then I write.

The Harry books are supposed to be full of surprises, but I tried to make sure that they unfold in a realistic way. The characters are allowed to act out of character and show hidden facets because that's what people do from time to time.

How did the idea of the Harry books come to you?
In a flash. Boy, doesn't know he's a wizard, sent to wizard school. That was the nutshell. I started thinking what wizard school would be like and I got so excited about it.

For Hogwarts school you've put ingenious twists to features that are standard in conventional boarding school stories. What do you think is the appeal of boarding school stories?
In fiction, boarding school comes over as a surrogate family. The pupils are with their contemporaries and free of their parents and the guilt attached to upsetting them. I'm comprehensive educated - I've never even been inside a boarding school. I was asked recently whether I would have like to have gone to one. No, definitely not! But if it had been Hogwarts - yes, like a shot.

When new pupils arrive at Hogwarts they try on a magic hat to determine which school house they should join: Gryffindor, Slytherin, Hufflepuff or Ravenclaw. Slytherin is associated with warped wizards. When people put the hat on they hear the hat's voice. Harry doesn't like what he hears about himself, he finds it disturbing.
What I'm working towards here is the fact that our choices rather than our abilities show us what we truly are. That's brought out in the difference between Harry and his arch enemy, Tom Riddle.

In Chamber Of Secrets, Harry is told by the hat that if he goes into Slytherin he will become a powerful wizard. He chooses not to do that. But Tom Riddle, who has been twisted by ambition and lack of love, succumbs to the desire for power. Though he's supposed to have died years before, his malign spirit manipulates events through an enchanted diary.

Where did you get the idea for that diary?
My sister used to commit her innermost thoughts to her diary. Her great fear was that someone would read it. That's how the idea came to me of a diary that is itself against you. You would be confiding everything to pages that aren't inanimate.

Did you have any inkling that Harry Potter And The Philosopher's Stone would be so successful?
I remember one day I'd been writing for hours and when I stopped I was buzzing with caffeine ... I was walking down the Bridges [the road outside Nicolson's Cafe where Rowling wrote the first drafts] thinking, 'The difficulty will be in getting this published. But if it's published, there will be a handful of people who really love it'. And then I thought, 'Oh come on!'. As it turned out, far more than a handful of people liked it. It has sold so well that I can now write full-time, and that's always been my life's ambition.

Do you put images from your dreams into your books?
Ideas come from all sorts of places and sometimes I don't realise where I got them from. A friend from London recently asked me if I remembered when we first saw Hogwarts. I had no idea what she was talking about until she recalled the day we went to Kew Gardens and saw those lilies that were called Hogwarts. I'd seen them seven years before and they'd bubbled around in my memory. When Hogwarts occurred to me as a name for the school, I had no idea where it came from.

I found myself thinking about the wardrobe route to Narnia [in the CS Lewis series including The Lion, The Witch And The Wardrobe] when Harry is told he has to hurl himself at a barrier in Kings Cross Station - it dissolves and he's on platform Nine and Three-Quarters, and there's the train for Hogwarts.
Narnia is literally a different world, whereas in the Harry books you go into a world within a world that you can see if you happen to belong. A lot of the humour comes from collisions between the magic and the everyday worlds. Generally there isn't much humour in the Narnia books, although I adored them when I was a child. I got so caught up I didn't think CS Lewis was especially preachy. Reading them now I find that his subliminal message isn't very subliminal at all.

Really, CSLewis had very different objectives to mine. When I write, I don't intend to make a point or teach philosophy of life. A problem you run into with a series is how the characters grow up ... whether they're allowed to grow up. The characters in Enid Blyton's Famous Five books act in a prepubescent way right through the series. In the Narnia books the children are never allowed to grow up, even though they are growing older.

I want Harry Potter and his friends to grow up as well as older, though I'll keep it all humorous, well within the tone of the books. I want them eventually to be truly 17 and discover girlfriends and boyfriends and have sexual feelings - nothing too gritty. Why not allow them to have those feelings?

What were your favourite books as a child?
Noel Streatfield's Ballet Shoes. Elizabeth Goudge's The Little White Horse. That was my favourite childhood book. I adored that book. I also loved Paul Gallico, especially Manx Mouse. That's a great book. Gallico manages the fine line between magic and reality so skilfully, to the point where the most fantastic events feel plausible.

Who are your favourite writers now?
Jane Austen, Nabokov, Colette. Of contemporary writers, I think Roddy Doyle is an absolute genius.

People sometimes compare you to Roald Dahl.
I've been compared to him more than anyone else. I take it as a compliment. There are similarities in our humour sometimes. Charlie And The Chocolate Factory and James And The Giant Peach are brilliant, but he's not one of my favourite children's writers. Our writing is really quite dissimilar. My books are ultimately more moral. An unfashionable word, but there you go. They're not moralistic, but there is often a good-versus-evil subtext. They're not absolutely black and white, though. Harry breaks a lot of rules. He's not good in the Enid Blyton sense.

I enjoy your slow-fuse jokes and word play.
My ambition is to write books that the reader won't necessarily get completely at the first reading. I read my own favourite books over and over until they fall apart, literally. I've gone through three copies of Emma - they get dropped in the bath and the covers fall off, and I've had to replace them. Nothing makes me happier than when a child brings me a copy of Harry that looks appalling - it proves they've read and read and read it.

Do you read serious books about magic?
I'm not a New Age type - not really into crystals. But through reading I know a ridiculous amount about magic. Some of the spells in my books are ones people have genuinely believed in. I find books about magic fascinating, but sometimes it's absolutely hysterical the things people believe.

What do you feel about having had such quick success as a writer?
I'm still slightly stunned to be living the life I've always dreamt about. At least once a week I get a funny shiver down my spine.