Quite a bit of the casting is imaginative...
The actress playing Sybil (Lara Rossi) is far too young and slim. Sybil should be operatic in every sense. Rossi is great but too delicate for the role. Sybil needs substance!
(Incidentally my mum has a friend who would be a perfect Sybil - good presence, excellent carrying voice, very jolly hockey sticks, constantly garbed in old but v expensive tweed and waxed jackets.)
I feel the actress playing Angua is giving her too much confidence; in the first book she's comparatively timid isn't she (the attitude comes later)? And where is her hair? I love short hair on women and have quite a similar pixie cut myself but Angua needs a mane!. She also looks a bit too petite for the role (in my opinion).
Cheery should be wearing armour and have a beard. And less makeup in the first book; as with Angua, that comes later as I recall.
They'd better dye Hugill's hair for Carrot.
Richard Dormer as Vimes might work.
James Fleet could work for the Archchancellor but I always picture him more like James Cosmo. The Sky adaptation of Hogfather had quite a good Archchancellor.
I like Anna Chancellor as an actress but Vetinari is obviously a man in the books.
Honestly I'm not sure I'll be watching much of this!