HE HAS been stripping naked on the London stage every night for three months as the sexually troubled boy in Equus. Still, Daniel Radcliffe is touchingly coy when I ask him about Harry Potter's first screen kiss.

As readers of JK Rowling's novels know, Harry Potter and the Order of the Phoenix is the one where the 15-year-old boy wizard starts grappling with manhood. In the film, the fifth in the blockbuster series, Radcliffe (whose actual age is 17) has to act out this rite of passage with the actress Katie Leung, 18, who plays Harry's new love interest, Cho Chang. "We shot it for a day," Radcliffe tells me, with a shy smile. "We did about 30 takes. Katie and I were both a bit awkward and nervous at first, but once we got into it, it was fine."

The kiss was possibly even more of an event for David Yates, the British director of the latest instalment, and his crew, many of whom have worked with Radcliffe since he was 11 years old and appearing in the first film. "They were gathered round the monitor and needed to see this moment," Yates says. "They had seen Dan go through all these changes as he'd grown up - he is like a son to them, so they got quite emotional."

If Harry Potter has come a long way as a character, so too has the industry that surrounds Hogwarts' star pupil. Before the latest film finished shooting earlier this year, I visited Leavesden Studios in Hertfordshire to talk to Radcliffe and Yates on set. The studio, a former aircraft hangar which was converted into a film facility for the James Bond film GoldenEye 12 years ago, is now the permanent home of Harry Potter. Warner Bros has a further six-year lease on the studios to complete the final two films with Radcliffe.

On my visit, I am allowed to wander through the sets - a spectacle in itself. It's no surprise that there is talk of creating a theme park or permanent exhibition out of it all when the series ends.

I walk through the atrium to the Ministry of Magic, which is modelled after a Victorian Underground station and is where the new film's climactic battle takes place. I pass on to the Hall of Prophecies, the Room of Requirement, Dumbledore's crowded study, and the Court of Justice, where we first meet this film's arch nemesis, Dolores Umbridge (played by Imelda Staunton). The detail in each construction is remarkable. Away from the sound stages is Privet Drive, where Harry lives with his aunt and uncle, and a chapel-like building attached to Hogwarts that plays a key part in the drama.

Each film employs hundreds of crew members and actors for each 11-month shoot. The budget for The Order of the Phoenix is estimated at £150 million. The prop department is now the biggest in the world, and some of the sets - notably Hogwarts Great Hall - are life-size constructions which have been used in each film. The massive hall has a floor built of original York stone. The benches and tables in it were distressed to look ancient for the first film. Six years later, they are now genuinely distressed after being used repeatedly by the cast and 320 local schoolchildren, shipped in as extras to play Hogwarts students.

Radcliffe has grown up on this set, but still, he tells me, nothing quite prepared him for the demands of the fifth film. "It's not physically challenging like the fourth one was, but in terms of the acting it's a lot harder."

His first snog is by no means the only hurdle the character has had to get over: in this chapter 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 that 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 until now 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 won critical acclaim.

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 round, like a kind of banned substance. It's very flattering."

Yates is a far cry from Chris Columbus, who made the first two films: he is a Hollywood A-list director, better 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."

Edinburgh-based author JK Rowling has also announced that the final book in the series of seven, Harry Potter and the Deathly Hallows, will be published on 21 July this year, 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 whom 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.

The story so far
THE PHILOSOPHER'S STONE: After 11 years of misery living in his relatives' stair cupboard, Harry is tracked down by Hagrid, who tells him he is a wizard. Shy and nervous, he begins his Hogwarts education. THE CHAMBER OF SECRETS: Having beaten the evil Lord Voldemort, he is now firm friends with Ron and Hermione, and becomes a hero of the Quidditch team - and has to rescue Ron's little sister Ginny.

THE PRISONER OF AZKABAN: Harry discovers Sirius Black, a criminal on the run from Azkaban prison, holds a link to his murdered parents. Black becomes the father figure Harry missed growing up. THE GOBLET OF FIRE: At the moment of his victory in the harrowing Triwizard Tournament, Harry finally meets Voldemort. Jealous of Hermione and Viktor Krum, he starts to develop a crush on Cho Chang.

ORDER OF THE PHOENIX: Expelled from Hogwarts, 15-year-old Harry is reunited with Sirius and other friends to fight against a conspiracy. One of his team is Cho, and the older girl begins to return his affection.