Brad Pitt or Blonde Prat as I like to call him as Greebo??????
Right, got that one sorted out. Nick Brimble as Greebo - he was Little John in Robin Hood: Prince of Theives
Jason Isaac could be a good Vetinari if Jeremy Irons wants to give up the part.
I think that Emma Chambers would be a great Magrat, Jane Horrocks was the voice of Magrat in the C4 Wyrd Sisters cartoon.
If they were doing Wyrd Sisters as movie then maybe Maggie Smith would be a great Weatherwax, she's played some really gutsy parts in the past, just watch Keeping Mum where she plays a Homicidal maniac if you don't believe me.
Television maybe Annette Crosbie (the one who played Victor Meldrews long suffering wife in One Foot in the Grave).
Miriam Margoyles would be an excellent Nanny Ogg in either.