Radcliffe considered celebrating his legal majority when he turned 18 in July 2007 by buying a sensible car like a Toyota Prius or a Golf GTI. Instead he decided to invest in art, falling for a blue ink drawing on white paper, called Mona D, Mary and Me, by the New York-based conceptual artist Jim Hodges.

"The dealer said they wanted to sell it to a more prestigious collector and Jim got word of this," Radcliffe said. "Turns out he's a massive Harry Potter fan and insisted they sell it to me.

"Ever since then I've been really good friends with Jim and his best mate, Tim, a photographer. And they are two gay guys, artists, in New York, and they introduced me to these amazing, crazy, mad, weird, extraordinary people. I was immediately embraced by the New York tranny community."

He offers a useful tip for any other actor hounded by paparazzi and fans.

"If you want to go out on the street without being recognised, without even being looked at, go out with a 6ft 8in beautiful transsexual," Radcliffe said. "No one gives you a second glance."

"There have been people who have tried to exploit me. You get chancers out there who just want to make a quick buck, but as long as you tune into them and who they are ... The best thing I've learned is, if you're going out, never go out alone - you leave yourself vulnerable. If you've got someone else there you trust, they can say, be wary of that person. I probably used to be too trusting of people."

"When you're in the position I'm in, you have two options: you can either shut yourself off from everybody, from the world, and not live a full life. Or you welcome everybody into your life and occasionally somebody will try to take advantage. And I'd much rather be that person who lets people in. Because, as an actor, people are your greatest resources."

He's published some poems under a pen name, and although he doesn't tell me what it is, he provides so many clues even Dobby the house-elf could solve it. It seems to be Jacob Gershon: Jacob is his middle name, Gershon the Jewish version of Gresham, his mother's anglicised maiden name. Modern poetry and free verse "irritates me", he says. "I love people like Simon Armitage. He has such an immaculate grasp of metre and rhyme, if he wanted to do poems like that, he could. But sometimes free verse, for me, is for people who can't do structure. And when I don't write in form and metre, I become unbearably self-indulgent. It's what Robert Frost said: free verse is like playing tennis with the net down."

Why does he like writing poetry? "As an actor, there is room for a certain amount of creativity, but you're always ultimately going to be saying somebody else's words. I don't think I'd have the stamina, skill or ability to write a novel, but I'd love to write short stories and poetry, because those are my two passions. There is an art to a short story. I love Raymond Carver, and Chekhov - without making myself sound more highbrow than I am!" he blusters, a reminder of the public schoolboy he was, on and off, until the age of 17. "I watch Britain's Got Talent like the rest of us."

The film also marks Harry's second kiss, with Ginny Weasley, sister of best friend Ron. Was that enjoyable? "It was quite weird for me because I've known Bonnie [Wright, who plays Ginny] since she was nine and I was 11. Very strange. But we got through it. It was good. And it'll get a bit of a cheer from the Potter fans. But I have to say, today I saw playback of Ron and Hermione's kiss [from the final book], and it is easily, from what we've filmed so far, the biggest moment in all the films. It is," he says approvingly, "a great kiss."

He pauses when I ask if he's happy with his performance. "Six is a very hard book to film, because it was essentially a lead into seven, but no excuses. I think I came through OK. I know I have a lot more to give than I do in six. And what's great is that I did Equus on Broadway between six and seven. I feel I've developed a lot in that time."

Alan Rickman, Severus Snape in the Potter films, was a big help on Broadway. He cut short a holiday in Connecticut to visit Radcliffe and give him some pointers on stage presentation "that absolutely saw me through the last six weeks of the run" - how to be still, exploiting his "quite short and compact frame". Radcliffe says he used to "struggle" with Rickman: "I never used to know when he was joking or not. I think I took a lot of his sarcasm seriously. But recently I've woken up to it and he's actually a great guy."

He proceeds to tell me the people he "absolutely loves" on set: Rupert Grint, who plays Ron Weasley, and Emma Watson, who plays Hermione Granger. "They are, to all intents and purposes, my brother and sister." Are they all best friends? "Probably not, only because we don't see each other out of filming. But someone like Tom Felton, who plays Malfoy, I'd count among my really good friends. I went to the cricket with him on Sunday."

But Watson seems to be enjoying all the opportunities for photoshoots and red carpet premieres. "Yeah, totally, but she's much more natural at them than I am. She's more suited to being able to talk to anyone - I get very nervous about those events. She's been photographed at a lot of [fashion] things, and I think that's a world she's very interested in. I've seen some of the clothes she's designed and [they're] very good. She's very clever. Do you know her GCSE results?" His eyes boggle: "I was thrilled with mine - seven Bs, two As and an A*. I think Emma got three As and seven A*s - she's incredibly academic, it's frightening. Me and Rupert to all intents and purposes dropped out of school. And she's going to Brown." He shakes his head in admiration of Watson's place at the US Ivy League college.

And finally, Radcliffe admits that as a boy actor he's had some "quite sexy mums over the years. Jamie Lee Curtis in [big screen debut] The Tailor Of Panama and Emilia Fox [in David Copperfield]. Both good," he says eagerly. He asks if I've met Rowling. "She is fantastically attractive. Very, very beautiful. And so intelligent, it's frightening."