Book four explores a lot of themes, some we've seen before in Chambers of Secrets, about prejudice. Is that something you've been wanting to explore?
From the beginning of the Philosopher's Stone prejudice is a very strong theme - and I think it's plausible that Harry enters the world - that's how I wanted it to be - he was quite wide-eyed about it, everything will be wonderful in this world, this is the place where those sort of injustices didn't happen and then he finds out that sure enough it happens.
And it's a shock to him like to everyone else and he finds out that he's a half person within the confines of the world. To a wizard like Lucius Malfoy, Harry will never be a true wizard because his mother was of muggle parentage.
So this is a very important theme and I always knew - well obviously I knew I've been trying to do it for 10 years now - yes so that becomes stronger and stronger.
Well I think it is often the case that the biggest bullies take what they know to be their own defects, as they see it, and they put them right on someone else and then they try and destroy the other and that's what Voldemort does.
And that was very conscious - I wanted to create a villain, where you could understand the workings of that person's mind.
And Harry, as you know, from book four, is starting to come to terms with what makes a person turn that way. Because they took wrong choices, and Voldemort took wrong choices from a very early age - he decided young what he wanted to be.
Was it difficult balancing the light and dark in the book? You've got some very dark moments and some wonderful moments of humour - talking about Mad-Eye Moody, the man who can't tell the difference between a handshake and attempted murder and a slightly dodgy joke about one of the planets in the solar system.
Yeah it is slightly dodgy. I was surprised my agent let me get away with that actually because as I wrote it I thought she's going to pull this book but she really laughed at it, so she let it stand.
Is it difficult? No because my experience is in a very limited way that even when life is really not that bright people still laugh in the most tragic of situations, people still laugh.
And the ending of the book is actually very important to me because, as you know, Harry says - We're going to meet the past - that's what's so admirable about human beings that even when they are really against it, when they are really in the direst of situations there is still humour, there just is, you will find that almost everywhere, so that's quite important to me.
Why was it important to show some of the strange friendships developing in this book?
Well in book four, for me, Harry, Ron and Hermione, all of them, are really starting to find their own identities and that means, in their various ways, facing up to the things that have been imposed on them by their parents or school.
For Harry that's facing up to his fame, really facing up to it for the first time because he's been put into this situation where he will, for the first time, really get the weight of outside interest. So that's scary.
Ron has to deal with his jealousy - he's made friends with the most famous boy in his year and that's not easy, it's not easy to be in that situation. And Hermione gets a political conscience. Hey!
Is this your idea of Hermione lightening up as you've said before?
No she will.
She didn't seem that light to me she was quite radical.
Yeah, she's a good girl Hermione. I agree with you she's not that light in this book but people made the mistake - when I was writing book four - of assuming that my answers related to book four, there are another three books to go.
But in some ways Hermione has - she's more of a rule breaker now, where her convictions are concerned she's prepared to do stuff that she's really not supposed to. So, in that sense, she will lighten up, I promise you, I did in the end.
Last time I spoke to you there was another Weasley coming in this book....
I know I'm sorry about that. What happened on book four and one of the reasons why it was easily the most difficult to write, which had absolutely nothing to do with Harry being famous or me being famous or anything like that, the first time my plan fell down ...
The famous plot hole. I got halfway through my plans and realised there was this huge gaping hole in it, there's two - it just didn't meet and that was entirely my own fault, I should have had the good sense to go through it very, very carefully before I started writing but I hadn't.
So I'd written what I then thought was half the book it turns out to have been a third of the book before I realised that this wasn't going to work, so I had to do an enormous amount of unpicking, and in the unpicking process I'm afraid the Weasley got [draws finger across her throat] ...
Will we be seeing her again?
It's possible, I really like her as a character but with my plot being quite intricate in the context of what I'm dealing with I'm not sure that she'll fit anywhere else, so she'll be the character that might have been.