There are two roles I could see Tom Cruise doing; either Larry Fowler or Madrigal Raith. The man can actually act, as evidenced by his role in Tropic Thunder. Nothing else though.
While I still stand by my earlier suggestion of Timothy Dalton as Nicodemus, Alan Rickman would also do a damn good job.
As for other characters...
-Terrence Stamp for The Merlin.
-Adrien Brody for Kincaid (I'm basing this on his performance in Predators, which proves that not only can he play a badass convincingly, he can do the gravely, Clint Eastwood-esque voice that Christian Bale fails at as Batman.)
-Eddie Izzard as the voice of Bob. I think he'd be a pretty good fit for Butters, but I'm not sure how well he can sound american, since he sounds
extremely British.
-Julian Richings fo Grevane. Grevane's described as pretty gaunt and corpselike already, and he definitely fits the bill. (He's Death on Supernatural, if that helps.)
http://www.imdb.com/name/nm0724995/-Danny Trejo for The Red King. Yeah, I know that the RK is supposed to be kinda short, but whatever. We need someone who can be intimidating as the RK, and he qualifies.