The Sunday Herald has an interesting interview with Gary Oldman from Harry Potter and the Prisoner of Azkaban in which he discusses why he took the job, how he portrays Sirius and how he feels about book 5.
Oldman on why he took the role:
Well, I needed the work – that’s the honest answer. I haven’t worked for a while, a couple of years. So I thought it would be nice to get back to work and earn some money. Pay the bills. Also, it’s prestigious, it’s a phenomenon, so you’re not just making a movie. It’s joining the family of some kind of cinematic dynasty I suppose. It is interesting to be a part of that. The material was good, the director was interesting and it is nice to actually be in a movie that my kids could see.
On playing Sirius:
I’m playing him as a bad guy and then you make that twist. It’s a bit like playing Shakespeare. Shakespeare doesn’t really write subtext, you play the subtext. You do it all in the line, I’ve not embellished it with a whole lot of psychology. It’s a certain technique to playing it, a certain style. I appear, you think I’m the bad guy and through the course of the scene you realise, “oh he’s not that”. But when you see me, I have an energy and intensity and the audience, not being familiar with the book, would still think that I‘m actually after Harry Potter and that I’m presumably in cahoots with Voldemort. I guess what I am saying is you play him as a good guy but he has been wrongfully accused and imprisoned.