Scene: a frozen lake in a Scottish forest. Beneath a grey, menacing sky, Harry Potter stands dirty and bloodstained, with a broken voice and a wonky wand. For the tenth time, he has to run down a mock hill in Shepperton Studios, point his wand, and shout, "Expecto Patronum!" at some mist, which one day will contain computer-generated Dementors, but for the moment contains nothing at all. Still, Daniel Radcliffe manages his intense Pottery stare yet again, despite the fact that the light on the end of his wand isn't working.

"Wasitawright?" he inquires. "Yes," says Alfonso Cuarón, the director of Harry Potter and the Prisoner of Azkaban. "Dan is never off his mark," he adds, generally. Then he looks at the shot on video replay. "No more new things. Forget the light on the wand," orders Cuarón in his Spanish-English. But the robot camera on a crane is casting a shadow on Radcliffe's face. They'll have to do the shot again, for the eleventh time. Radcliffe waits, drumming a complicated rhythm with his wand. Cuarón gives some final suggestions to Radcliffe: "Don't breathe. Don't lean. Don't nothing. Don't act."

Someone is sent to scatter bags labelled "Kosher salt" on the hillside to improve frostiness. A scenery man goes into the bushes to twitch them, spookily. Radcliffe skips happily up the hill, ready to run again. He grins at Hermione, Emma Watson, who is in the background talking in sign language to the crew. She yawns. Radcliffe spins his wand like a cheerleader. Across the fake lake, Gary Oldman, who plays Sirius Black, appears to be having a little picnic from a brown bag. Soon he lies down on the hill, apparently asleep.

Welcome to the third year at Hogwarts, where Harry Potter confronts an escaped prisoner and the soul-sucking Dementors who are sent to guard the school. And welcome to the slow torture of acting in a major motion picture with far too many special effects.

Off set, Radcliffe, 14, is grey-faced and looks shattered. "When you're doing it, it's not tiring at all, with all that adrenalin pumping around. But it hits you when you stop, like jet lag. The worst thing is waiting around for the really complicated shots: you have to be focused all the time, or you won't do yourself justice. I'm so tired by the end of the day I just get home and fall asleep."

Remember, Radcliffe is doing two jobs, unlike the adult actors. The children spend nine hours at the studio - four on the set and three to five being tutored for school. Despite this, Radcliffe appears to be more balanced and cheerful than most teenagers. Does he not get bored? "Not really, I keep going by watching films and listening to music. I play all kinds of rock."

Emma Watson, also 14, has arrived with Radcliffe to face the press pack. She likes Justin Timberlake and Brad Pitt. "What is it about Brad Pitt?" asks a journalist. "How long have you got?" replies Watson. She says she doesn't mind being recognised in the street.

"I don't mind it either," adds Radcliffe, but he was surprised when 3,000 girls met him at Tokyo airport. "Amazing, if slightly scary."

Radcliffe, Watson and Rupert Grint, who plays Ron Weasley, spend a lot of time in each other's company. "Between movies we didn't see each other," says Watson, "but we stayed in touch." Asked about the next movie, she snorts and says, "Not thinking about it."

They talk about the animals in the film. Radcliffe says there is a particular owl - Hedwig is played by two owls - called Gizmo that he gets on with. "Owls are so cool." Watson is pleased with her cat. "Cruikshanks, I love her." Radcliffe makes a yuck face. "That cat's horrific. It looks like it's been hit by a pan." Watson pokes him, hard.

Robbie Coltrane, who plays animal-loving Hagrid, has a cheerful loathing for his film menagerie, particularly bloodhound Fang, who has halitosis and drools, so is trailed by an animal trainer with a dishcloth. "You're acting your arse off, then the dog moves, so they print the shot where the dog stays still. I give off alpha male vibes and it does something to them. Animals are overrated."

Coltrane lights up another Dunhill. He's quite tough with

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