It's a distinctly private pleasure, listening to Stephen Fry reading the [Harry Potter]books. Some of us grown-ups like to just settle down on the sofa and concentrate, while others prefer to listen while chopping carrots in the kitchen, wallowing in the bath or burning up the motorway.

But of course we are trespassing: these recordings are really intended for children. There is one little girl who likes to be fully kitted-up in her witch's costume before the tape is switched on, while J.K. Rowling's own daughter, Jessica who is, her mother admits, "completely obsessed" with the tapes prefers to be tucked up in bed before the story begins.

This is actually a bit of a problem. When Rowling was writing the fourth and latest of her books about the young wizard, the creative peace of her evenings was disturbed by the sound of Fry, on tape, reading the earlier novels to her sleepy seven-year-old. "As I told Stephen, it was not very comfortable! Harry seemed to be everywhere, while I was trying to concentrate on his new adventures." But she understands and shares Jessica's addiction. "I've got, ooh, loads of talking books. My sister laughs at me and says it's a very old lady habit, but I don't care. I've always enjoyed being read to".

She is absolutely delighted with Fry's reading of her own books. "I love it," she exclaims, "I absolutely adore it. When his name was first suggested I said, 'Oh {please}let it be him!'" Asked to define what it is about his performance that so delights her, she explains that she thinks he's got "a very, very attractive voice. It's quite patrician but there's a touch of anarchy that's just right for the books. And he's not overly actorly about it: he's like somebody's uncle who's really good at reading aloud. It's a very intimate feeling."

Fry himself thinks that part of the stories' appeal is their concretely realised world: children love "knowledge and sovereignty over an imagined world. Hence the appeal of things as diverse as Sherlock Holmes and Tolkein..." However, even these two are overshadowed by the bespectacled wizard. "Neither of them offer such a mix of humour, fright and fun." Like all Harry Potter fans he has trouble identifying a favourite. "I love being Dumbledore because, like all actors, quiet power and dignity and greatness are my favourite parts ... But I think all round it's a toss-up between Hermione and Ron. Though, of course, let's not forget Harry himself. Or Hagrid. Or Professor McGonagall. Or Mrs Weasley. Damn it, I love them all," he sighs happily.

The latest much longer book, [Harry Potter and the Goblet of Fire,]was released on 2 April on cassette, in two boxed sets. Like the others, its production has been watched over by Helen Nicoll, founding-mother of {Cover to Cover}. This company has, for nearly 20 years, specialised exclusively in unabridged readings. It was therefore an obvious choice for Rowling, who is said to allow no abridgement of her books. She's happy to endorse this: "I just said, let's do the whole thing or not at all. I get enormously frustrated if books are cut and I've yet to meet a kid who was sorry we did it that way".

One result of this attitude was the clearing of Radio 4 schedules on Boxing Day, so that Fry's reading of [Harry Potter and the Philosopher's Stone]could go out, complete and uninterrupted. Helen Boaden, Controller of Radio 4, took a gamble on this but it paid off probably because Rowling's style is so compelling that those who came to mock stayed to pray. The broadcast attracted 4.5 million listeners, including one old boy who had first written to complain at the prospect, but then handsomely wrote again to say that he'd loved it.

The film of the first book, now in production, is remarkably faithful to the original. "There are a couple of very tiny cuts but I'm amazed at how much they managed to get in. Mind you, if they ever film the fourth book they'll be in trouble. There are a couple of scenes in there...goodness knows how they'd do them." And she laughs. "That'll be their challenge, won't it?"

Such is the power of the human imagination that no such problems exist in the audio world. Instead, for Stephen Fry, the challenge is to remember and reproduce the voices he has devised for each of the large cast of characters in every successive book. To remind him, Helen Nicoll arranged for a CD of snippets of the early recordings to be produced for him, but he seldom needs it. The voices aren't based consciously on anyone he knows. "They sometimes accidentally come out as similar to certain schoolmasters, friends and so on, but that isn't the intent," he says.

His one regret relates to the stern Professor McGonagall: "I regret not giving [her] a Scottish accent. It's all the fault of her first scene in [Harry Potter and the Philosopher's Stone]where she's a cat, and I started the cat with one voice having forgotten that she was going to transform back." Apart from this he seems to know instinctively how the characters should sound. Jo Rowling agrees enthusiastically. She cites the example of the delightful, disaster-prone character called Hagrid, about whom she worried. "It was at the beginning. Stephen was recording the first day of the first book and I was flying down from Edinburgh to hear him and the plane was delayed. I was desperate to tell him that Hagrid was from the West Country lest he made him a Glaswegian or a Scouser and almost the first thing he said was 'I've done Hagrid from Somerset. Is that all right?' I was so relieved".

When we met, she had yet to hear the latest reading, but she was confident of the result: "I'm really looking forward to hearing [The Goblet of Fire] as interpreted by Mr Fry."

©BBC Worldwide/Sue Gaisford