I never noticed this before, but Randall Flagg has some similarities to one of my favorite villains: Richard III (as Shakespeare wrote his story). In Richard III, the eponymous Richard can talk anyone into anything. In The Stand, Flagg has a similar ability—though his persuasion is aided by the fact that he can scare the bejeezus out of anyone. But the big similarity I see is that it all starts to go wrong once these characters loose their abilities to control other people’s behavior.
I was also surprised to learn, from reading his entry in Wikipedia, that Flagg appears in other King books. It’s sort of being in on an inside joke, being able to spot characters from other books. And it’s probably why I like Jasper Fforde’s books so much. From what I understand, though, King’s books are very intertextual. There are frequent references to other villains, other towns, other heroes. After a while, it starts to look like King’s books are all part of one big, scary-as-hell story. I know I’ll never read every thing he’s written (because I like a good night’s sleep), but it would be interesting to see how it all works together.