But Gary Oldman, Radcliffe says, 'was the person I was probably most starstruck about meeting. We're now really, really close.'

'I felt in a way that [in the new film] I got to play the character rather than the situation,' Oldman says with noticeable satisfaction.

But beyond that, 'there's something incredibly magnetic about Gary. I just think it's his intensity. It's a constant process of refinement when you work with him. Some actors will do a fantastic first take and after a couple of days, start to get tired. With Gary you could do 100 takes and every time he's trying to get it better and better. He's fearless.' [Dan Radcliffe]

I ask Oldman if this is fair comment.

'Um.' Pause. 'Well it's very sweet of him.' Pause. 'Yeah, I guess that's pretty... Hmmm. Not as fearless as Mr Radcliffe. Because I don't know if I could have done what he's done, and in particular what he's done recently in the theatre, in Equus.'

'I saw Dan recently, in Equus. I sat there and thought, God, it would be nice to do this again. But I get over that pretty quickly,' he says, laughing again.

However, he concedes, he does hanker after the company of theatre. 'That's what one misses with the movies. That's what's been great about doing three Potters. Because it really is pretty much all the same people. The directors and crew change, but the hair and make-up and costume people and the cast - that's a nice thing. That is what you get from the theatre. It's a conspiracy of friends.'

I ask David Heyman, producer and custodian of the Harry Potter films, what he thinks Oldman has brought to a franchise already studded with stars.

'A wonderful combination of danger and heart,' he says. 'Gary has a youthful exuberance which is just beautiful. And you feel that he has lived. It's etched all over his face, and etched all over his performance. He brings a real depth and humanity to Sirius Black, who is a complex character. He's been locked up for 12 years in Azkaban prison, and now he's escaped. He is in many ways the father Harry never had.'

'Gary's an actor's actor, but also a producer's and a director's actor. He comes prepared, he's collaborative, he's passionate. I've heard the stories - "Oh Gary..." - but nothing could be further from the truth. Gary is a hero. You just want to see him do it more. To have him flexing those not-inconsiderable acting muscles in Harry Potter is a treat.'