There is a new interview with David Thewlis who plays Remus Lupin in the Harry Potter films.

Question: And do you have much to do in Half-Blood Prince?
Thewlis: Oh, in Half-Blood Prince? No, not a great deal. About the same as Order of the Phoenix. No, I’m not central to it in any way. It’s a few weeks work. Maybe a little more than in Order of the Phoenix.