The guy who plays Hiro Nakamora in Heroes. He has the right look about him, that sort of complete idotic innocence.
I really like the idea, but isn't he pretty young to be playing Twoflower (who, if memory serves, has grown up children)?
Given that Simon Pegg as Rincewind is perfect casting, it's a shame that Nick Frost couldn't do Twoflower.
When I was reading night watch I kept imagining a really good casting for Vimes, but now I can't think of it, that really annoys me.
I suppose the other question is, if you had to choose one book to make into a movie which would it be?
Men At Arms, but then I've got a soft spot for the Watch books. Vimes is a tricky one to cast; I think he's probably Pratchett's best developed character (with a proper arc and everything, which is more than can be said for a lot of them) and consequently is much harder to "type". I'm thinking maybe Ralph Fiennes (but all skinnied up and dowdy looking) or Daniel Day Lewis (though he might be too intimidating to play Vimes, whom I've always seen as sort of small and wiry - hence his tendency to fight dirty).
As for the first few books, I guess most of the wizards could be pretty much played by anyone since it's mainly old-man-hair-and-beard acting that'd be called for. The Librarian I guess would have to be CG and animal noises, though I really like the idea of them doing a South Park and getting a really famous actor into a recording studio just to say "ook" in several different ways. I find that in the early books a lot of the characters are just too much of fantasy parodies to be really interesting to think much about, but Granny Weatherwax will probably generate a lot of opinions. Personally I'd say either Meryl Streep (just because she seems to be able to do pretty much anything) or Emma Thompson (I suppose that's the obvious choice, but I can see it working).
As for directors, I'd love to see what Terry Gilliam would do with Anhk Morpork or how a director like David Fincher would approach the darker comedy of the Watch series.
Oh, and Gaspode would obviously have to be Colin from Spaced. Duh.