Can J K Rowling weave a new kind of magic after Harry Potter? So do novelists feel they use a different part of their imagination when they switch genres? Anthony Horowitz says: "It's about entering a mindset. It's not about the choice of language. It's not even about the content, although of course both of these come into consideration. It's more fundamental. It's about attitude. Children's books have a purity about them, a simplicity. There's less clutter, less to worry about. When I sit down and write a book, I don't consciously think about my audience... which is to say I didn't deliberately make The House of Silk more adult and the Alex Rider series more childish. But I was always aware of a sort of perception, an intuition, that separated the two. Yet while some adult novelists have turned to writing children's fiction as a kind of sideline (perhaps believing, mistakenly, that children's literature is easier to write), few children's authors do what Rowling has done, which is to switch to the postlapsarian world of adult fiction. Some, like Anthony McGowan, whose outstanding young adult novels such as Hellbent, The Knife That Killed Me and (for younger readers) The Doughnut Diaries have reaped praise and prizes, express a wistful regret at not making a go of adult fiction. "Hellbent was the first book I wrote - but I wrote it as an adult book," McGowan says. It didn't get a publishing deal, but my agent suggested I try to write something more commercial, so I wrote Stag Hunt - an out-and-out adult book. I then revisited Hellbent, and turned it into a teen book by taking out some of the more overtly adult content. I found myself a teen writer, without ever having planned it. The fact that there are things you can't say - both in terms of language and content - does mean that in some ways writing for children or teens is more limiting. I do still hanker after adult fiction. Given the choice between the Booker and the Carnegie, I'd plump for sharing a table with Zadie and Co. It seems strange, given how long-lived a good children's novel can be, to hanker after the brief acclaim that adult fiction usually brings, but McGowan's feelings are shared by many other "crossover" authors. The late Joan Aiken, whose Wolves of Willoughby Chase sequence continues to delight generations, also wrote adult novels largely inspired by Jane Austen. Though charming and accomplished, they lacked the rich vocabulary and wild imagination that was unleashed in her novels about the dauntless Dido Twite. When I interviewed her, many years ago, she couldn't quite understand why her children's books were so much more highly regarded. Changing genre is never to be undertaken lightly. Often, no amount of talent in one genre can make up for an author's deficiencies in another. Perhaps the most famous instance of this is Henry James. Despite being acknowledged as "the Master" of the novel, he longed for success as a West End playwright; David Lodge's Author, Author recounts how cruelly his expectations were dashed by a bored and irritable audience. In James's case, the lure seems to have been (in part) financial gain - something that is no doubt behind the efforts of some adult novelists who have switched genre. Yet sometimes good authors can feel themselves trapped in a particular kind of story, which they resent. Conan Doyle, plagued by the creation of Sherlock Holmes, thought his historical novel, The White Company, was his best work. Julian Barnes's pseudonymous Dan Kavanagh detective novels and Ian McEwen's book for children, The Daydreamer, have earned them fewer sales and less glory than their literary works. Yet switching genre can also produce exciting results. Susan Hill followed much-lauded realist novels by brilliantly reviving the ghost story with The Woman in Black. Hill's imagination, always drawn to tales of sorrow and cruelty in small masterpieces such as Strange Meeting and I'm the King of the Castle, segued quite naturally into gothic tales of supernatural revenge, which is perhaps why it proved so enduringly successful, not only as a novel but as a long-running stage production and, more recently, a film (starring the Harry Potter star Daniel Radcliffe). Whether Rowling will have played to her own strengths is what we are all waiting to see. Will she once again have created a small community, a plot powered by mystery, murder and very British characters whose comical eccentricities overlie serious moral dilemmas? Bereft of overt magic, will she continue to cast a spell over millions of readers as they prepare to read The Casual Vacancy before bedtime? Who knows, but we can all hope for a little touch of Harry in the night.