Granny Weatherwax is a very hard one to cast - there has to be the power, but without grandeur, and the tragedy of her unconsummated love in her youth for Mustrum Ridcully. Glenda Jackson or Helen Mirren? Maybe Vanessa Redgrave (again, if she can lose the RADA accent).

No, George, you don't want to be her, she's a virgin remember?

Maybe Daryl Hannah for Angua.

LTS for Perdita?

And Twoflower? If you don't like Jackie Chan for Rincewind, how about him for Twoflower?

And you can't have Wilfred Bramble for Albert, he is Cohen the Barbarian to the life.