Was the experience of working on "Harry Potter" what you expected?

I had the best experience of my working life making these two films. The English crew was the best crew I've ever worked with; the actors were completely professional - there's no star nonsense to deal with, even though they are stars. It's been an amazing experience. But I can't do the third for my own family obligations. I've put blood, sweat, tears, everything into these two movies, and I wouldn't be able to give the kids [Daniel, Rupert and Emma] the same amount of energy on the third film, and they deserve that. Having a relationship with these three kids has got to be one of the great things that's happened to me in my life.

How did you come up with the look of Dobby?

He's described in the books as being a little shrivelled. This is a guy who's had a really difficult life, so we didn't want to portray him as being cute and cuddly. He's basically worn the same clothes all his life and been a servant. You have to fall in love with his inner-character.

We had some concerns about doing a CG character, based on some other CG characters in the past who I won't name who were kind of annoying, and we didn't want to be anything like that. What we tried to do was make a character that the audience could love - that was the big challenge - and actually feel for at the end of the film. Part of the credit for that goes to ILM for coming up with a great, realistic-looking character with fantastic skin textures and all of that stuff. The big part of the credit goes to Dan Radcliffe, who was able to focus on a green ball on the end of a stick for all of those sequences. All of our animators said they'd never seen a kid perform like that, with that kind of focus. They said people who are twice his age couldn't focus the way Dan did. So Dan made those scenes work.

How do you as a director ensure the film's child stars have a normal childhood?

For these kids, I suggest they look at the lives of all the child actors who preceded them in terms of where it didn't work out and they say to themselves, "OK, what didn't work that time, why did these kids have problems as they got older, what went wrong?". Look at that, and then look at the people who were successful - people like Jodie Foster, Ron Howard - who took what they learned from this experience and went on to make not only great lives but great careers for themselves.

My advice to these kids is, when you reach a certain point when it's not fun any more, where you feel like you're doing it as a job, or someone older than you is saying you have to do it - walk away, quit, because it's not important. The only thing that matters is your friends and your family. They know it, you can tell these guys have not changed, the key is they have to keep a level head. They all have great parents, and I believe their futures are pretty secure and safe.

When will we see the third movie on screen?

I don't know if we have a date yet. I know the studio is talking about a date, but the questions is whether we'll make it. I don't know. We're not ready to commit to that yet, we just want to take a little more time with pre-production. And again we've learned a lesson with visual effects - we want the visual effects to be as great as they can be, so we need to take a little more time in post-production.

"Harry Potter and the Chamber of Secrets" opens in UK cinemas on Friday 15th November 2002.