As usual, the movie hews closely to JK Rowling’s source novel while exercising some cinematic licence. In the book, events in the outside world are often relayed via Potter-verse newspaper The Daily Prophet.

On screen, there’s less reporting, more action. Rowling’s passing reference to the fall of a ‘Muggle’ bridge, for example, has turned into an opening set-piece salvo: large, loud and London-based. “Let’s just say the Millennium Bridge doesn’t get well treated,” quips producer David Barron.

But one book-to-script tweak had to be unwritten when Rowling unexpectedly outed one of her creations. “There was a line of dialogue where Dumbledore refers to the opposite sex in a romantic way,” says Heyman.

“But after the first read-through, Jo told us that this wasn’t in keeping with his character! So we made the change…” The Hogwarts headmaster’s sexuality isn’t made an issue of in the film. “Although there is his obsession with knitting patterns,” smiles Yates.

Dumbledore is very much to the fore in Prince, training Harry for battle while heading towards his own date with destiny. “It’s a big movie for him,” says Heyman, lauding Michael Gambon’s commitment to the role. Yet, as with the character, there are revelations to be had about the veteran thesp…

“The secret about Michael Gambon is that, as much as he’s revered, he’s also the most unprofessional actor in the word,” joshes Radcliffe. “He never takes anything seriously, which is why he’s great fun to work with.”

The actor spins anecdotes of Gambon playfully bombarding him with rubber rocks – and real ones – on set. "So I threw bits of stuff back at him. It was really infantile. And he’s a ‘Sir’, isn’t he? ‘Sir’ Michael Gambon, my arse!”

Much as he enjoyed mucking about with Gambon, Radcliffe missed another of the series’ father figures – Gary Oldman.

“It was sad to do the first film without Gary,” he sighs. “He was a bit of a mentor who let me know when I was crap and when I was good. Actors aren’t always the most generous, so when someone like him said ‘That was really good,’ to a young actor like me, it was amazing. So I kind of missed that.” 

Still, the pair keep in text contact; and besides, there are other mates around – like fellow mainstay Tom Felton.

As bad boy Draco Malfoy, Felton has skulked on the sidelines for the last eight years but in Half-Blood Prince, Harry’s classroom arch-rival finally slopes into the spotlight.

“This is my favourite of the films,” says Felton, as cheery as Draco is sneery. “The first one I’ve worked on from beginning to end, not just a month here and there.”

Felton’s big moment comes when he and Radcliffe square up for a scrap that plays like something out of Bond or Bourne. “It was great,” raves Felton.

“We spent a week in this misty bathroom set, rolling around in the water having all sorts of fights. It’s a longer sequence than it is in the book.” There’s blood – but also some fleshing-out.

“This film develops the character further than before,” he says.“We take the chance to explain why he’s such a complete cretin, that he’s not just doing it for the sake of it.”