I was so fearful of breaking the spell

Mike Newell had already directed a string of hits, including Four Weddings and a Funeral. But, he tells David Gritten, the latest Harry Potter film was one of his biggest challenges

In his time, the veteran director Mike Newell has tackled a remarkably wide range of film genres, and has yet to be defeated by any of them. He can do costume drama, as the success of Enchanted April demonstrated.

A Mafia story? It was Newell who brought together the combustible combination of Pacino and Depp in Donnie Brasco. And he was the steady hand on the tiller of the phenomenal Four Weddings and a Funeral, a deceptively effortless-looking romantic comedy which, lest we forget, did not direct itself.

Newell's chronic reluctance to become typecast, then, makes him a difficult man to present with a new challenge. But, two years ago, it happened when he agreed to become the first British director of a Harry Potter film.

He admits that Harry Potter and the Goblet of Fire, the fourth in the series, was a first for him in many ways. He had never made a hugely expensive studio movie. The fantasy action-adventure was new to him, he lacked experience with computer-generated imagery, and he had never directed a sequel in an existing series. One assumes that this individualist director might feel hemmed in by the existing constraints imposed by the success of the preceding Harry Potter films.

"Oh, of course, things made me fearful," says Newell. "It's a colossal franchise, so you need to not screw it up. There's all that special effects stuff, and a very big budget."

As he tells it, the film's producer David Heyman and Warner Bros executives asked him to read the book, and see if he could find a way of making one film out of it. "They'd been thinking of turning it into two films," Newell recalls, "because it's a big book, a 750-page doorstopper with a rambling structure. It's a classic example of what JK Rowling does best - she's a brilliant comedienne, and a terrific writer of characters."

Yet Newell decided that essentially The Goblet of Fire was "a God-given thriller". Set once more in Hogwarts school, its story is based on competitive struggles - from the Quidditch World Cup to the Triwizard Tournament, in which a champion from each of three wizard schools strives to fulfil perilous tasks. Harry (Daniel Radcliffe) is too young to enter, but to his bemusement his name is selected by the enchanted Goblet of Fire. The film's climax pits Harry in one-on-one combat with his malevolent nemesis Voldemort (Ralph Fiennes), who murdered his parents 13 years previously.

"I was explaining my idea of the story to Dan, and he said, 'What have you been watching?' I told him, paranoid thrillers: Three Days of the Condor, The Parallax View, North by Northwest. They're all about people who don't know what's happening to them.

"I told him specifically to watch North by Northwest, because there you are, it's a sunny afternoon, you're happy with your life, but suddenly stuff starts happening, and then you're up against the bad guy, who had plans for you all along. That's exactly what happens to Harry Potter in this book.

"I found you could make a movie so driven by Voldemort's agenda that you'd be in a very stressed, creepy world from the beginning, and you'd then stalk Harry all the way through. The story has a wonderful classical shape. So I went to work with a will."

Newell concedes that this fourth Harry Potter film is even more intense than its immediate predecessor, The Prisoner of Azkaban. "I think it's darker, and crueller. But you can't keep making these films for little kids. It's clear that JK Rowling is now not writing for little kids. She's moved on up, and you'd better do the same."

This darkening of tone shows itself in the Triwizard challenges, each of which recalls some archetypal bad dream: dodging a fire-breathing dragon, risking being trapped under the waters of a huge lake, and navigating a maze with tendrils that reach out, touch and finally devour.

The other shift of tone is that the film's three young protagonists - Harry, Hermione Granger (Emma Watson) and Ron Weasley (Rupert Grint) - are now all teenagers confronting new emotions. In different ways, each one deals with a potential romance. One wonders if Radcliffe, 16, Watson, 15, and Grint, 17, might be feeling restless or sense that they are growing out of these films that have dominated almost half their lives.

"They live a curious existence," says Newell, almost in a whisper. "But they all have good parents, and they manage to keep a certain distance between their real lives and their on-screen lives. They have a lively sense of the ridiculous, especially Dan. He knows it's weird to have Japanese girls lying down in the road in front of his car, so he can stop, get out and propose marriage to them. Which has happened.

"They're all strong characters, too. You might think Rupert was rather dozy, but he's a glorious comic. He'll be a name."

All three, Newell thinks, have an ability to separate themselves from the wilder excesses of celebrity. "Emma goes back to school and ordinary life, whereas Rupert's given up on school and makes other films. So he does it that way."

Rowling, he reports, was rarely on set and maintained a hands-off policy. "Everyone assumes she's some kind of dragon, but she insists the books are the books, and the films are the films. As long as the films don't contradict the spirit of the books, she has no problems. We had to take a couple of liberties with this story, but she was fine with it."

Newell has certainly delivered a film along the lines Warner Bros would have wished; hundreds of millions of dollars will doubtless roll in via box offices. He recalls the experience fondly now, but admits: "It's a very odd way of working, and not a way I enjoyed at times. It tends to be controlled by the hours the kids can work: four and a half hours a day. So you have to work with doubles, stand-ins, and any notion of spontaneity in setting up a scene is out the window. And also all the film's huge set-pieces have to be storyboarded. I hated storyboards for the same reason."

Understandably. Newell, 63, is of a generation of filmmakers such as Stephen Frears, Michael Apted and Richard Eyre who worked long years in British TV drama before moving into film. They tend to share freewheeling instincts and an ability to turn their hand to any genre.

Clearly, Newell had to curb those instincts on the Harry Potter set, though a sly signature touch can be seen in his casting of two favourite actors and collaborators: Miranda Richardson (Dance With a Stranger) who plays Rita Skeeter, a scurrilous gossip reporter, and Brendan Gleeson (Into the West), who is Mad-Eye Moody, an eccentric Hogwarts professor.

Still, Newell would be prepared to immerse himself in the Harry Potter process for another two years, under the right conditions. "I'd love to direct the film of JK's last book," he admits. "That would have to be an interesting story. I imagine there'll be a long queue of applicants, though."

• Harry Potter and the Goblet of Fire is released on Nov 18.