Christopher Walken as Vetinari.
Sean Connery as Ridcully.
Hugh Laurie as Vimes.
Ewan McGregor OR Brendan Fraser (love 'em both) as Carrot.
Definitey Nick Frost as Fred. Definitely...
Emma Thompson (and her 'fat suit' from Love, Actually) as Sybil.
The fellow (sorry) who played Nobby in Hogfather as.. well, Nobby.
And Jack Nicholson for Lord Downey. I know, I know, Downey's character doesn't appear in Guards! Guards!, but still...
Disclaimer: I refuse to have cable television. If it was free, I wouldn't have it, but considering what it does cost (about 100US per month), absolutely not!
because our PBS (public broadcasting) station does not exist anymore, British films & shows in the US are limited to what I can scout in the video store. Hence, my unfamiliarity with many of the actors mentioned.