Goblet has some extravagant set pieces, including a Quidditch World Cup match and a wizarding tournament between Hogwarts and two rival schools that involves a dragon, a killer hedge maze, and an undersea kingdom. How did you handle that last one?
We built a huge tank — far and away the biggest tank for filmmaking purposes in Europe. Dan [Radcliffe] and the other children involved had to do two seemingly simple things: keep their eyes open and their faces mobile enough to express emotion. They're fish, you see — they've been magically given gills, so they can breathe. So there could be none of that ''screwing up your face and looking panicky because you're running out of air'' stuff. You had 12 seconds before we had to send people down with air masks. We were limited to extraordinarily short stretches of time for each take.

You tapped Ralph Fiennes for Voldemort. Why?
I wanted Voldemort to be a tremendously charismatic man, and Ralph has tremendous charisma. I wanted to have someone who was entirely cold in his heart, and I knew Ralph could do that very well from Schindler's List. And he's a star. He has all the presence and command that a star has. That is very useful for this part. You can't have an undercooked Voldemort.

One of the reasons producer David Heyman wanted you for the job was because you were British, and therefore familiar with the nuances of British school life.
For the other directors, it was research; for me, it was instinct. But it was more than that. It's my view that children are violent, dirty, corrupt anarchists. Just adults-in-waiting. I was very anxious to break [the franchise] out of this Goody Two-shoes feel. There was one scene where two characters have to fight. They simply weren't fighting the way I knew they could fight. So I said, ''Which one of you wants to fight me?'' One finally volunteered, and we started to fight, and he was still pulling his punches. I said, ''Don't do that!'' So I banged his head on the floor and he banged mine, and we got there. I got up from that with a banged rib.