There just has to be a place for Benedict Cumberbatch
somewhere. If only he was a bit older I could see him as Vetinari.
MongoGutman, I like Robbie Coltrane for Fred! But I'm not sure about Robert Carlisle for Moist - I always see Moist as having one of those inoffensively handsome but fairly forgettable faces - Craig Parker, maybe.
Carlisle as Rincewind, now that I could see. He does put-upon, harried and persecuted so well!
What about John Hannah for Lord Downey? He's got a great contemptuous sneer!
My William de Worde would be Lawrence Fox. He's so tall, but he has this habit of hunching his shoulders and folding in on himself. I could see William being like that.