Welcome back to Harry's world

His first snog is not all he has to cope with: Harry is racked by the guilt he feels over Cedric Diggory's death at the end of the last film, and must come to terms with the death of Sirius Black (played by Gary Oldman), his mentor and godfather, at the hands of evil death-eater Bellatrix Lestrange (Helena Bonham Carter).

"Doing the real isolation and anger and hurt he feels has been a challenge," Radcliffe admits. "Thankfully I've never been bereaved in my life so I don't know what it's like, but it is incredibly difficult to act the Sirius stuff."

Potter Five is a more emotionally complex film than the previous four, and producers David Heyman and David Barron have continued their tradition of enlisting a new director each time the series takes a new turn. Following Alfonso Cuarón's dark fourth film, Yates, who hasn't made a single feature film, nevertheless brings to the fifth an impressive list of successful, grown-up television work - State of Play, Sex Traffic and The Girl in the Café, all of which attracted critical admiration.

His is a name to conjure with among the Hollywood elite, and State of Play - Paul Abbott's edge-of-the-seat political thriller from 2003, which starred Bill Nighy as an editor trying to link two murders - is being remade this summer with Brad Pitt in the lead role and Kevin Macdonald (The Last King of Scotland) directing. "That went down particularly well in the States," Yates says. "JJ Abrams [who created Lost] said he was passing episodes around, like a kind of banned substance. It's very flattering."

Yates is a far cry from Chris Columbus, who made the first two films: a Hollywood A-list director known for family movies such as Mrs Doubtfire and Home Alone. "The work I've done has a large emotional content," Yates says, " it's very character-driven, and the core part of this fifth book is a very emotional story. Harry Potter has no mum or dad and he clings to this relationship with Sirius Black. So I think what they were looking for when they asked me to do this was to make sure we had a story that felt emotionally rich. It's rawer, edgier, darker and tougher. My contribution will be to grow the films up, as the books are growing up."

The ageing process has been a delicate issue all round, but particularly for the young stars of the series, Radcliffe, Rupert Grint (who plays Ron Weasley) and Emma Watson (Hermione Granger), when Warner Bros wanted them to commit to the sixth and seventh films.

Radcliffe and Grint reached a deal fairly quickly, but Watson was slow to sign. "Daniel and Rupert seem so sure," she told US magazine Newsweek last year. "I love to perform, but there are so many things I love doing. Maybe that sounds ungrateful. I've been given such an amazing opportunity, but I'll just have to go with the flow." She did finally succumb, for a reported $4 million a film, but not before Grint was quoted as saying that he and Radcliffe were "distant from her now. We don't text or talk to her when we're not filming."

JK Rowling has also announced that the final book in the series of seven, Harry Potter and the Deathly Hallows, will be published around the world on 21 July, one week after the fifth film hits cinemas. A major character is going to die, she warns the legion of Potter fans.

"I'll go with whatever she writes," Radcliffe says, philosophically. "I think it will be quite exciting if the only way that Voldemort could be killed is if Harry dies as well. There is a very strong connection between them. But I don't know yet. I've had talks with [Rowling] but not about that ..."

As to his relationship with the other child actors in the film, Radcliffe is open and polite. "I think when you're thrown together with two people who you haven't known previously and you end up spending the next six years together, you had better get on. We're very lucky that we do." He pauses before adding, "I haven't really seen them outside of the films."

Watching Yates directing the three young stars in take after take, there doesn't seem to be any hierarchy or tension among them. Their respective guardians - Radcliffe's father included - crowd around the monitor watching their wards, who cannot perform without an adult relative on set until they reach 18.

I wonder what it has been like growing up in a bizarre extended family on one of the most elaborate movie sets in cinema history. "It's something that hasn't really been seen before to this extent," Radcliffe says. "We're a group of kids growing up on-screen. It's sort of an interesting cinematic experiment."

Indeed it is. But we'll have to wait until next month to find out how it's progressing.

Harry Potter and the Order of the Phoenix is released on 12 July.