The return of the casting agent!
I just found out that the BBC did indeed make a TV miniseries of Titus Groan and Gormenghast and that Christopher Lee did indeed play the part of Flay. Richard Griffiths was Swelter. Cora Groan was played by Zoe Wannamaker. Stephen Fry was also in it, but playing a part I haven’t come across and wikipedia has nothing to say on the subject.
The Edward whose name I couldn’t remember is Edward Furlong.
Of course, it’s not just books you can do this to. But books are good because you don’t have a visual image of your character imposed upon you. So it is easier to find actors who fit. With cartoons or videogames or whatever, you are constrained by the person looking at least a bit like the character… Remaking films with new actors is another good source of idle speculation. “If I were remaking Bridge on the River Kwai, who would I have play Saito?” for example. (Beat Takeshi, obviously.)