“There really is stuff in here, especially toward the end, that will scare young children,” cautions Radcliffe with relish, his voice newly resident a few octaves down the scale. “I’m really pleased they haven’t veered away from any of the darkness.”

Widely considered, by those who consider such things, the finest of the Potter books, The Goblet of Fire revolves around a Triwizard Tournament between schools. A risky form of sports day involving dragons, mazes and confining pupils to the bottom of a lake, it also masks a heinous plot by Harry’s nemesis Lord Voldemort to trap his young foe. “That was absolutely the hardest thing I’ve had to do in the series,” says Radcliffe of the crucial confrontation.

“I’m sort of slightly worried about it, because it’s what the four films have been building up to. Harry is meeting the person who killed his parents and he wants to kill. But I can tell you Ralph Fiennes is fantastic.”

Fiennes, to general applause, has taken up the ghoulish visage of Voldemort. But his entrance from the fringes of the stories is merely the climax of a much more forbidding adventure all round — a film, for the first time, confronting the permanency of death.

“There is a real attempt to define something about the character in terms of being mythic,” says Mike Newell (veteran of Four Weddings and a Funeral and Donnie Brasco), the first British director enticed to dip into the pressure-cauldron of the Potter franchise. “You put him through the kinds of tests that by surviving and succeeding turn him into a hero.”

Newell has pursued this edgier, darker feel because he realised that at the heart of the fourth book was a “cracking thriller”. Cut away most of the decorative flimflam (Rowling’s doorstop runs to 636 pages) and you are left with the kernel of something classic. “It is like North by Northwest, this classical thriller where the good guy catches up bit-by-bit, until to his horror he finds himself absolutely at the centre of the bad guys’ machinations.

Thus, it has to use the visual rules of a scary movie.”

Although parts of the new film cleave pretty damn close to being an actual horror movie, this is not the reason why those two famous faces are caught in such rapt attention. Today, on set, pirouetting their way down the Great Hall are a dozen very chic French babes, from the magic school Beauxbatons, led by one particularly comely blonde. This is Fleur Delacour (Clémence Poésy), Beauxbatons’ haughty champion.

Of all the perils the no-longer-a-boy wizard has confronted since his debut in 2001, none is as formidable as this — girls! He and Ron are even required to land a date for the upcoming Yule Ball. Yikes! Fighting Voldemort is a doddle in comparison.

“It is something I think Mike will bring the best out of,” laughs Emma Watson, returning as Hermione. “There is this very funny scene with Ron asking Fleur to the dance and he just sort of shouts at her.”

“There is a real change with the characters growing up now,” adds Grint. “More of a teenage aspect to it. Ron is more moody. He has some nasty experiences asking some girls out, and he and Harry have this huge row. It lasts for most of the film.”

Regardless of whether it might exclude the smallest of Potter fans, to find such funny and painful aspects Newell knew he would have to push his young actors to find “a human truth”. Or, more bluntly: “to really bloody act!” Thus, before filming, he instituted a series of acting workshops for the junior end of the cast. These include new recruits Poésy, Katie Leung (as object of Harry’s affections Cho Chang), and Stanislav Ianevski. He plays hunky Viktor Krum, from the Bulgarian school Drumstrang, a “kind of boyfriend” to Hermione. Also attending were the old hands Radcliffe, Grint and Watson.