they won't cast someone else because christopher nolan said
that the joker's role is supposed to be started and ended in this movie, as he comes in and makes his way out of the story (whichever way that may be) and its not essential for him to come back.
so. things i've noticed over the dozens of times of seeing it:
we see two-face in the movie. not the acid (which i hear is a flammable liquid which catches on fire, instead of acid. which is way crazier) but when you hear harvey dent say "you either die a hero" (at 2:02) you see him in a car with a gun, and you see the edge of the burn damage, and his discolored suit. apparently harvey dent becomes two-face earlier on in the movie than most people suspected (not like a lame end-of-movie cliffhanger meant to be continued in the next movie. we see a lot of two-face
we see batman landing on a moving van at 1:06 and what i think is the scarecrow in the driver's seat. could just be a masked joker-cronie, though.
we see the joker burning a bunch of money at 1:31. he's so ****ing insane. goes through so much being a bank robber and stealing money, and just burns it all hahaha.
and the obvious... you see rachel dawes get thrown out a window by the joker. maybe katie holmes didn't like the idea of her character being killed off?
