David Yates is spending four days in purgatory. It's in his contract. Warner Brothers has booked a whole floor of Claridge's Hotel in London to promote its GBP75m, fifth JK Rowling adaptation, Harry Potter and the Order of the Phoenix. The company has filled room after room with increasingly wild-eyed talent from the movie, including hormonally charged teens Daniel Radcliffe (Harry) and Emma Watson (Hermione). Every few minutes, they send in another journalist to ask the same questions. How many takes did you need for Harry's first kiss? Tongues? Can you show me how you did that thing with the wand? How can you justify not having a quidditch scene, when there was one in the book, damn you?

Warner won't let anyone out until every last Texas blogger and Czech cinephile leaves happy in the knowledge of precisely what motivates Professor Dumbledore in his beardy dotage, and exactly which manifold architectural influences were key to the construction of Hogwarts school. "What's the weather like out there, mate?" asks Yates. "We don't get to see it in here."

The world's press has one question for Yates: Who the hell are you? Here's my version: Remind me how a nobody from St Helens with a track record of making fine, if low-budget, TV dramas such as Paul Abbott's State of Play - your several awards notwithstanding - gets to direct the fifth instalment of a global franchise that has grossed GBP1.75bn? No offence. "None taken, mate," says the gentle-voiced 43-year-old. "Many nights I've been lying awake thinking, 'Of all the films in all the world, why am I doing this? How did that happen?'"

But surely you were a fan of the books, or at least the previous films? "No. Hadn't read a word. It wasn't part of my world," says Yates. So how did it happen? "I was walking in Cornwall when I got a call. They sent me a script and I read 20 pages and thought, 'This isn't going to work. This isn't me.'"

Yates's dithering reminds me of Daniel Craig contemplating becoming 007. It's not just all the bother of having to endlessly answer stupid media questions, but the risk of never being taken seriously as an artist again.

He became captivated by Rowling's world of wizardry: "The problem was I was dropping in on series five. So I went back and read all the novels and then I was hooked. I spent the next year working on the script." What, apart from unprecedentedly large pay cheques, was the attraction? "The characters. Harry struggling with who he is and the powers he has. Also, I was captivated by Dolores Umbridge, who is a mean, but apparently sweet, magic teacher [played by Imelda Staunton with gale-force gusto], who is sent to spy on Dumbledore. She's a sadist and a bully. There are kids in the audience who have had horrible experiences with teachers or other adults. So they'll feel connected to the story. If any bugger turns up." Oh, they'll turn up all right.

"Another thing that got me was how our Muggle world sits right next to the world of wizards. It's there but we don't know it. That had wonderful possibilities." Two of his film's visual flourishes grew from that thought: a London terrace splits open to reveal the house where Harry Potter meets the Order of the Phoenix; and a red phone box slips under a road near the Westminster parliament to the Ministry of Magic, whose subterranean atrium is thrillingly decorated in 30,000 black, shiny tiles.

But why Yates? David Heyman, producer of all the HP films, says he chose Yates because of his track record: political dramas such as Sex Traffic, and his direction of Anthony Trollope's The Way We Live Now. "David is a fantastic actors' director," says Heyman, of the graduate of Beaconsfield's National Film and TV School.

Heyman also reckons Yates "has shown he can handle political subject matter in an entertaining way. This is not a political film per se, but the politics of the magic world are very much at play here. We thought David could handle that brilliantly."

Surely that can't be right, I say to Yates. How can someone with a TV background possibly deal with scenes involving weird CGI horses flying above the Thames at night, a giant created by "motion capture" (whatever that is), and the mayhem of a 20-minute finale set in a room of crashing crystal balls? Not to mention the massed egos of Britain's leading thesps (Emma Thompson, Helena Bonham Carter, Michael Gambon, Robert Hardy etc)? Plus the fact that most of Harry and his gang have grown up with their roles and might eat you for breakfast? Wasn't Warner risking a great deal by putting you at the wheel of this obscenely lucrative juggernaut?

Didn't it worry them that your only previous feature film was The Tichborne Claimant, nine years ago, and, with all due respect, that didn't have any CGI dementors, nor a full-on wand fight between Gary Oldman and Ralph Fiennes, nor every kid in the world wanting to see it?

"That's not quite right," says Yates, quietly. "This is really a story about growing up into an adult world riven between good and evil. That's about actors and script rather than big effects. The effects are the easy part anyway. I'm known for intensity and emotion, and I think I've given them that. It's a bit edgier and rawer than the others, arguably." Insofar, that is, as a Potter film can ever be raw or edgy.

Yates cites George Lucas's 1973 film American Graffiti as an influence. "It's one of my favourites because it deals with teenage angst and rebellion. All of that is relevant to my film." But there's no magic in American Graffiti. "The magic in HP5 is partly a metaphorical struggle to do with growing up."

This isn't as wild a notion as it sounds. My favourite lines are when Harry says: "I feel so angry all the time. As if I'm being bad." Suddenly, there is a connection between Potter's burgeoning sexuality and the magical Manichean struggle. It would have been nice to have more of this. So is Potter going bad? No, it's just his turbulent hormones making him think so.

Which brings us to The First Potter Kiss. What a responsibility! "I know. The first thing I did was clear the floor. Otherwise, the prop guys, everybody, turns up for a good look. I got Daniel and Katie [Katie Leung, who plays Cho Chang] to talk about their first kisses. You have to do that to make the atmosphere intimate. I told them about mine and they told me about theirs. It was about them getting lost in each other, to get them to forget they were on a film set." The couple kiss in a magical room that senses how they feel about each other and so makes mistletoe grow above them. Personally, that would make me hurl, but it works for Harry and Cho, and for the people quietly sobbing around me at the preview screening.

It took 30 takes. And, presumably, a lot of lip balm. The kiss reminds me of what Lauren Bacall said to Humphrey Bogart after he embraced her in The Big Sleep: "I liked that. I'd like more." But there is no more. Harry and Cho never find themselves in another clinch, for all the film's expansive 138 minutes. "There'll be more of that in the next Potter," says Yates. "This film is about teenage rebellion. The next one [Harry Potter and the Half-Blood Prince] is sex, drugs and rock'n'roll."

Yates is to direct it, too, which shows Warner was happy with the expertly assembled, if bombastic, hokum that is HP5. "Yes, I was quite surprised they liked what I did.

I thought they were going to say, 'Change that, change this,' but they were lovely. Eight months of shooting and they came over once. They made a few suggestions and then left on the Warner jet." As you do.

Yates is already imagining how to adapt The Half-Blood Prince. "We start shooting in eight to 10 weeks. In the book, bridges blow up. I want to take down the Millennium bridge." What, don't you like Norman Foster's iconic bridge? "I love it. I just love the idea of destroying it."

What about the final Potter book, Harry Potter and the Gauntlet of Overkill? Sorry, I mean Harry Potter and the Acne of Destiny. No, that's not it either. Harry Potter and the Trumpet of Portentousness, Harry Potter and the Decanter of Doom? OK, it's actually called Harry Potter and the Deathly Hallows, and it's due on July 20. Does Yates fancy directing that? "I have to see if I'm still standing after six." Do you know how the final volume ends? "No. They haven't sent me an advance copy. I want to know if Harry gets killed. I'm hooked."

Finally, why did it take Yates so long to get into the big time? "Because I got sent rubbish scripts. Why would I want to direct rubbish films when I could work on a perfectly paced thriller by Paul Abbott, even if it is for telly? It was only with HP5 that I really got tempted."

I leave Yates in purgatory. Damn, I forgot to ask him the big question. Why no quidditch, damn you? "Two words," I can imagine him saying, in his gently mocking Merseyside accent. "Bor. Ring. How many quidditch games do you need to see at the pictures?