More character shading comes in the shape of flashbacks to Voldemort’s youth, magically conjured by Dumbledore as part of Harry’s training. Not easy, casting evil incarnate as a kid.

Luckily, it doesn’t sound like they have another Jake Lloyd on their hands… “It took a while,” recalls Heyman. “There was a lot of discussion
over how similar they should be to [adult Voldemort] Ralph Fiennes.”

In the end, they decided on Fiennes’ own nephew, Hero Fiennes-Tiffin, as the 10-year-old future dark lord; while Frank Dillane (son of Brit-actor Stephen) takes over for the teenage years.

“Both of them have this disquieting sense of darkness about them,” Heyman praises. “Frank projects a kind of superciliousness – he’s
very superior, very in control of the situation. And with Hero, as with Ralph, it’s all about the stillness… He evokes Voldemort so successfully, so creepily.”

The film’s visuals also play their part in establishing eeriness. “The choice of angles, the extreme close-ups, the pacing of the scenes,” recites Heyman, going on to laud director of photography Bruno Delbonnel, the French lenser responsible for Amelie’s lavish palette.

Half-Blood Prince goes for an equally heightened look, U-turning from the stark, dark timbre of Phoenix. “It’s very layered, incredibly rich,”
says Yates, before revealing that number seven – Harry Potter And The Deathly Hallows – will be more naturalistic “with loads of hand-held cameras”.

“I want to shake things up every time I go into this world. I like experimenting as we go along,” says the director, best known pre-Phoenix for the original BBC version of State Of Play.

Now, his rep rests on Harry Potter, which he’ll preside over until 2011, when Part 2 of Deathly Hallows hits cinemas (with Part 1 due late 2010).
Three previous directors have graduated from Hogwarts (Chris Columbus, Alfonso Cuarón, Mike Newell), so why did Yates decide to sign up for the long haul?

“It was around the end of Pheonix,” says the fortysomething, looking out over a forest set covered in imitation frost. “I got a taste for it. It’s a big, wonderful world to play in.

"You get all the resources you need to tell the stories, the people are terrific and the studio keep off your back. I was smitten with all those things combined and I wanted to be the one to see it through to its completion."