When Warner Brothers took out an exclusive lease on the studios ahead of the production of Harry Potter and the Philosopher's Stone at the turn of the millennium, that universe began to make its mark inside. Sets were built, costumes and creatures created, props stored. As we visit the studios in January 2008, 80 days into the production of this sixth and (kind of) penultimate film in the franchise, everywhere we turn past the perimeter gates there's a familiar Harry Potter sight to take in.

Leavesden Studios, the base for Harry Potter film production.

On a trip to one stage, away from the main complex, we pass giant serpent heads from Chamber of Secrets, chess pieces from Philosopher's Stone and part of the bridge from Prisoner of Azkaban. At one of the many prop stores dotted around the complex there's a London phone box from Order of the Phoenix and in the creature shop there are models galore used in the production of Goblet of Fire.

But the main focus of the studio right now is on Half-Blood Prince and RT has arrived about halfway through principal photography. Familiar sets - the Hogwarts Great Hall, Dumbledore's office, the Gryffindor common room - are fully constructed as they have been semi-permanently since the first film (in fact, the Great Hall set has lived on its own stage since 2000) and plenty of finished sets are in place for this movie. The interior of the Hogwarts Express train has just been used for a scene between Harry and Draco Malfoy, they've shot Christmas on the Burrow set - the home to Ron's family, the Weasleys - and they're getting ready to shoot some flashback scenes in an immaculately presented Potions classroom involving their new adult character, Professor Slughorn, who's being brought to life by Jim Broadbent.

Daniel Radcliffe and Michael Gambon share the screen alone for much of the film.

Half-Blood Prince really belongs to Dumbledore and Harry though, as the old wizard prepares his young protégé for his final steps on the quest to kill Voldemort. Despite the grandeur of the sets they've created -- including a giant green-screen warehouse full of crystalline rock that will form the backbone of the film's cave sequence -- it's a smaller scale than Harry Potter is used to, as actors Daniel Radcliffe and Michael Gambon spend much of the film alone with one another.

"The brilliant thing about [the cave scenes] was that we did them at the at the end of a three to four month period where Michael and I had been working together almost exclusively," Radcliffe tells RT. "For about the first two or three months of this film we were almost the only two actors in. The relationship between me and Michael had really built up over that time and got to the point where after three or four months we were absolutely ready to do the cave stuff."

Considering much of the action takes place at Hogwarts, where hundreds of students go about their business in the background, the change of scale was more akin to a play than a blockbusting movie. "There was that kind of dynamic where you get to know someone very, very well in a short space of time," Radcliffe continues. "Your relationship becomes based on certain things. I think mine and Michael's relationship is very much based on our senses of humour. I think I now understand the way he works more and I think we've become much closer through it."