Does the Discworld movie have the worst cast ever?

Rincewind needs to be played by someone much younger, and someone who has a much better comedic range than wise-cracking, cheeky Jason. I can think of two actors who would fit the bill.

Second place would go to current Doctor Who actor David Tennant. He oozes sarcasm and failure. As much as I dislike the stories and overall feel of the new Doctor Who, Tennant is perfect and he would make a pretty good Rincewind.

First choice though, would be an American actor who is often mistaken for being British - Alexis Denisof. He played the caddish chap who nicked Sharpe's wife in the TV series of Sharpe. He's much better known as Wesley Wyndham-Price in Buffy the Vampire Slayer and Angel. Absolutely spot on!

Twoflower definitely should be played by an Asian actor. I always thought that Twoflower represented the whole Japanese technology thing with the chest and his camera, but for the life of me, I can't think who to cast.

Following your suggestion of Chan, I guess a second pairing of Steve Coogan (Rincewind) and Chan (Twoflower) could work, but Chan does like to be up-front a lot and the story would suffer from Twoflower pushing Rincewind to the back.

Pete Postletwaite would make a brilliant Cohan the Barbarian though!! Or maybe Patrick Stewart - the man has hidden comedic tallent.

EDIT: An afterthought - if you chose not to cast Twoflower as Asian, Alan Davies would probably be good in the role.

David Jason was there to guarantee a big audience. God knows why, but the public seems to love him.