Smash hit

the Potter fans, too. "Adults come up to me on planes with the

books and I say, 'Admit it's for you or I won't sign it.' I've never read them, but my children have." Although against working with animals, Coltrane likes the children. "I've known the kids for three years now - bloody hell, they're not kids any more." "What have you

contributed to their education?" someone asks. "I couldn't possibly comment," says Coltrane, grinning, in his poshest Scots accent.

They are, as Cuarón says, great kids. "I received three kids who'd been in two big movies and learnt their craft. But they were also, at 13 or 14, starting to take themselves seriously as actors, and to understand the emotional aspect. Daniel Radcliffe has an amazing visual imagination. Kids are good at make-believe: the werewolf is there. And for the first time, they could say the lines in one shot." He smiles. "There's also a little uncontrolled energy coming in."

Cuarón seemed an unlikely choice for Potter III. Blockbusting Christopher Columbus directed the first two, but Cuarón, a hairy, rather radical Mexican in an orange jumper, had his doubts. "You had to be part of the franchise; I couldn't make the third one utterly different." His most famous film was the tender but raunchy road movie Y Tu Mamá También. "I came at it from a snobbish, authorial point of view. I was ignorant of Harry Potter. I didn't know I would spend a year pushing it in every territory. I hadn't read the books or seen the films. I just threw the script someplace. Then I read it, so I could say no. And I thought, 'Oh man, this is amazing.'"

The Cuarón Potter film is darker and more menacing, but so is the book. "I've changed quite a lot but no one will notice. I've made the Quidditch gear different, the uniforms darker, and there are more street clothes. I wanted to make the acting as naturalistic as possible, because if you ground the emotional reality, the magic becomes a matter of fact." Behind the forest stage set is a huge storyboard, with each shot cartooned; Cuarón leaves nothing to chance. He favours panoramic shots: "I wanted to keep the context present, use the background as a character."

"Chris and Alfonso are very different directors," says Radcliffe. "Chris was very high energy, which suited the first two films, but Alfonso's more laid-back: there's more emotional intensity." Cuarón points out that Potter III is about about a child trying to find his identity as a teenager, "and he discovers the bogeyman is inside himself and not in the closet. Becoming 13 years old, trying to understand yourself, is a rite of passage in every culture."

Gary Oldman comes in, emaciated, greasy-haired and dotted with runic tattoos. He's playing Sirius Black, the baddie-turned-godfather and says he likes the lack of "big sugary close-ups". Plus, it's nice to be in a movie his kids can see. "It's like acting in a very sophisticated pantomime. You are on the line in a very present way. Lickety-split, you have to get through it and there's no room for subtext. You can't complicate it, just play it with intensity." Oldman's only disappointment is that J. K. Rowling gives him the chop in book five. "Yeah, I thought I was on to a goldmine. Then halfway through the filming, the next book comes out and I die... Very disappointing."

So what else is new about this latest Potter? Fresh star material includes the Dementors, the Hippogriffs, the purple triple-decker Knight Bus, the Shrieking Shack, Julie Christie as the publican Madam Rosmerta, and Michael Gambon as Dumbledore.

Cuarón was worried, after the amazing special effects in Lord of the Rings, that the Dementors would be a letdown. "But they are more metaphysical than physical; they attack the mind, yet if you touch them, they disappear. So we started with black cloth swirling underwater, and created a computer-generated image from there."

Unlike most films sets which disappear after filming, there is almost a Potterdrome at Leavesden studios, north of London. The Great Hall is there, as are the classrooms and the Shrieking Shack. Hogsmeade village is permanently under snow, and even the rock is run through with the word Honeydukes in the sweet shop. One day a theme park will surely come. More than 1,000 people work on each film: a vast industry, as institutional as Hogwarts. The sets are set in stone, or wood, because they will be there for seven films, so confident is Warners of J. K. Rowling's imagination.

Cuarón has found it tough to carry the weight of the Potter institution. "It's not an exercise in ego, but an exercise in surrendering your ego to the material. The machinery is so perfectly oiled, so pleasant for your creativity, so instead of walking, you're trotting, then running, and it accelerates away from you. Suddenly you feel you're wearing a scarf and it's got caught up in the machinery and it's strangling you." This may be why Cuarón has decided directing one Potter movie is enough. n

Harry Potter and the Prisoner of Azkaban is released on June 4

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