In the last movie Harry seemed to face psychological and physical challenges, whereas in this movie the problems he faces are more intellectual, as he tries to piece the puzzle together. Does anything change in your approach to the character?

Daniel Radcliffe: I don't think a huge amount does change in terms of how you approach it. You come to every scene with the same basic tools. You come to it with knowledge of your character and knowledge of what their fundamental wants and needs are and how they would go about getting them. And you would just apply them to whatever situation they're in. The real challenge of acting for me, I suppose, is just getting to know a character very, very well and just applying what I know about them to every single scene. That's what it can be broken down to.

In the case of Harry in this film, his want - or his need - is basically to kill Voldemort and he realises that the way in which he'll do this is to become Dumbledore's favourite foot soldier. That's the role you see him gradually moving toward in this film.

He's preparing for the battle to come.

DR: Yeah, absolutely, he's preparing for the seventh [and eighth] film.

Has anything changed, for you, between the way David Yates has approached Order of the Phoenix and the way he's approaching Half-Blood Prince?

DR: I think David's a lot more confident this time around. He was brilliant on Order of the Phoenix, I think he did an amazing job, but the way he is on set now, I think he feels a lot more comfortable here. And that's understandable because it is quite an intimidating environment to come into when a lot of us have been here for seven years. I think he's been a lot more confident this time around.

Radcliffe with David Yates on Order of the Phoenix.

Harry doesn't really suffer Slughorn too gladly and yet I can imagine working with someone like Jim Broadbent is nothing but a pleasure - how has that been?

DR: Working with Jim is amazing just because he's such a brilliant actor and he's so immersed in the character which is always a treat to see. But, in a way, because he is so in character it kind-of makes it easier to act the way Harry does around him.

But I think Harry ultimately does like Slughorn. I think he thinks he's a very good person and his heart is in the right place but he's just very opportunistic and totally self-obsessed. I think Harry, in a strange way, finds that endearing and in an even stranger way perhaps actually likes the fact that Slughorn is fascinated by the aura of fame and glory that he sees surrounding Harry. And Harry also finds it, obviously, very useful in the film, because he has to exploit it.

How did you enjoy shooting the cave scenes with Michael Gambon?

DR: The brilliant thing about [that sequence] was that we did it at the end of a three to four month period where Michael and I had been working together almost exclusively. For about the first two or three months of this film we were almost the only two actors in. We did a few scenes with Rupert and a few scenes with Jim but other than that it was few and far between that you'd ever see any other cast members on set. The relationship between me and Michael 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.

It's almost like a play to be working so constantly with so few other actors.

DR: Totally. I didn't think of it that way at the time, but there was that kind of dynamic where you get to know someone very, very well in a short space of time and your relationship becomes based on certain things. I think mine and Michael's relationship is very much based on our senses of humour. We've got very similar senses of humour and I think when you know that you're both going to have to work together very closely doing very difficult stuff over the next few months you really need to very, very quickly find something that you can sort-of bond over. I've worked with Michael a lot over the last few films but more so in this one - by a long, long way - than I ever had before. It was great stuff.