Why I'm still in love with fairy tales

The author of His Dark Materials is excited to see his new story, I Was a Rat!, performed on stage in Birmingham

Red heads: Dodger Phillips and Christopher Dingli in 'I Was a Rat!'

It began with a picture in my mind, as most of my stories do. A moonlit doorway, an apprehensive old couple looking out, and a little boy in a ragged page's costume saying to them "I was a rat!"

What would happen after that I had no idea. I seldom do. But I could see ahead as far as the little boy sitting down at a table and trying to eat by putting his face into a bowl of bread and milk. I don't know if any children are still given bread and milk, but it was a staple of my childhood: there was something profoundly comforting about it, and I wanted to show the old couple (who by now were called Bob and Joan) knowing exactly the right thing to give this little lost boy who said he'd been a rat, and the little boy doing just what a rat would do when presented with it.

The old couple, of course, had always wanted a child but had never had one. This is a fairy tale, and very little in a fairy tale is new or unexpected or original. The couple who have no child but want one turns up in many of them. They turn up in one of the Grimm's very best, Hans-my-Hedgehog, and they turn up in another of mine, Clockwork, or All Wound Up, and here they are again, with the little boy who'd been a rat, and who certainly seems to be ratty in his habits.

One of the delights of fairy tales is this very lack of originality. It's similar to another quality they have: the characters in a fairy tale have no depth, no psychology. They don't fret about their feelings; they often don't appear to know that they have any feelings. Everything they do is on the surface, because the surface is all there is. Even when a character is sly and deceptive, we can see that immediately, although the other characters can't. Is there any reader who is deceived by the smooth words of the wolf in Little Red Riding Hood?

So in I Was a Rat! Bob and Joan are all goodness, the little boy is all puzzlement and honesty and wants to do things in the right way, if only he knew what that was, and the villains are all villainy in their various forms. After writing a novel, where you try and give your characters the semblance of depth and self-contradiction and the ability to reflect on their lives, it's immensely refreshing to go back to the fairy tale, where everything can be exactly as it seems.

But you can still be taken by surprise. A short way into this story, when I still didn't quite know where it was going, it occurred to me how the little boy had got into this predicament. He must have been one of the rats that Cinderella's godmother had changed into pageboys for the course of the evening, and he'd failed to get back to the coach at midnight, and remained as a boy. So it was a story set in the Cinderella world.

And there could be an infinity of those. The world in which Cinderella happens contains any number of possibilities. Suppose the royal orchestra is discontented, and go on strike for more pay: will the matter be settled in time for the ball? There's another story. Suppose the fairy godmother comes to the wrong house: there's another. Suppose it's Cinderella's sisters who are kind and helpful, and the fairy godmother who is devilish and cruel: there's another. If a story is a line connecting a number of events and situations, we can imagine a thousand events and situations in the Cinderella world and draw a million lines connecting them in various ways.

Some, of course, are much better than others, and that's where the skill of the storyteller comes in; but they're all possible. In this variation, the Cinderella figure comes in only briefly, because it isn't her story; but she's very important.

And now I Was a Rat! is on the stage. The Birmingham Rep, 100 years old this year, has allowed my story to be one of its centenary productions, which makes me very pleased. In my conversations with Teresa Ludovico of the Italian company Teatro Kismet, who has created this version, I was reminded of the great Italian tradition of the commedia dell'arte, where the masks have as little depth as the characters in fairy tales, but just as much vigour and narrative power. I'm hoping for absurdity, pathos, and a happy ending.

Philip Pullman's 'I Was a Rat!', Birmingham Repertory Theatre (0121 236 4455, iwasarat.co.uk) to 2 March; then touring until June