Hi there, newbie here....
Ok, so I have to agree with a lot of you guys on the choice of Maggie Smith for Granny. Nanny Ogg I could see Julie Walters or Miriam Margolyes. For Magrat someone kind of ordinary looking, not too pretty. Maybe Evanna Lynch (Luna Lovegood from Harry Potter)? As Verence I could see Robert Carlyle, but he'd also be a great Rincewind (or even Carcer).
Tiffany Aching is hard, cause I don't know any very young actors. Helena Bonham Carter could be a great Miss Tick though!
Vetinari: Alan Rickman or Ralph Fiennes. For Vimesy I can see Hugh Laurie or maybe Gary Oldman? As Lady Sybil I can only imagine Miranda Hart, I always picture her looking like that. Carrot is a hard one, maybe Colin Firth? Too old? Christian Bale possibly.... Nobby makes me think of Rowan Atkinson.
What would you guys think of James McAvoy as Carcer? Kinda innocent looking, mischievious...although I could also see him as Moist von Lipwig.
Oh, and I just thought Hugh Grant might make a hilarious Leonard of Quirm. Gosh, I could keep going for days, but I think it's time to stop. Make me stop!
I DO know how to spell Gonorrhea, by the way....erm. Yep.