More characters.
Agnes-and-Pernicia: I don’t know. We need a talented plump actress with really big hair.
Mr. Teatime:
Om:
Ponder Stibbons:
Mort:
I don’t have any suggestions right now. Someone else think up actors.
Since I agree with The Gaspode about using Rhys Ifans for Rincewind, I think Rowan Atkinson would do well either as Ponder Stibbons or the Bursar.
Rowan Atkinson for the Bursar, then. Ponder Stibbons needs someone who can do genuine competence. 
Hmph. Always pictured John Cleese as the Bursar and a younger Eric Idle as Ponder.
Ponder Stibbons - Rick Moranis?
Mr. Teatime - Robert Downey Jr.?
What about the dwarfs? Cheery Littlebottom?
‘The Truth’ reads like a screenplay - what about that photographer vampire? A hilarious part for the right actor.
Why do I suddenly see Anthony Hopkins as Vetinari?..Now THAT would add a whole new level, I think…
One of the MacGann brothers as Vimes, and how about Matthew MacFadyen as Ponder Stibbons?
Having watched HP&CoS again, I can see the actor who plays Lucius Malfoy in quite a few roles. I think he could maybe even do a good Veterani if push came to shove?