Director - Christopher Nolan

Originally, the Joker (Heath Ledger) was supposed to walk down the street while the explosion at the hospital began, get on the school bus during the scripted pause, and the bus would drive away while the explosion finished.
However, Ledger stopped walking during the pause and in a moment of improvisation began fidgeting with the remote detonator in a very Joker-esque manner - bringing a slight amount of dark humor to what would have just been a serious scene.