I see Nicholas (? - Rodney the plonker) as Rincewind.

Rickman is too charismatic and "forward" for Vetinari, but how about as the villain from Going Postal?

Carrot is difficult - maybe Kevin Costner, but perhaps a bit too old?

Keep the girl who did Susan in the TV Hogfather.

Agree Brian Blessed as Ridcully.

I see John Tams as one of the wizards - maybe even Vetinari, with a trim and hair dye.

Tom Cruse as King of the Dwarfs - or is that naughty?

Off the wall perhaps: Michelle Pfeiffer as Weatherwax.

But who is going to be Adora Belle Dearheart?