In this clip from a documentary about Ledger, his father looks through a journal he kept to figure out the character.
Watching this clip of Kim Ledger going through his son's "Joker diary" is a very emotionally confusing experience. On the one hand, I was filled with a sense of loss for such a great talent dying at the young age of 28. On the other hand, I was terrified at the stuff in the diary. It's like all of my nightmares put into one, and it explains why his version of the Joker was so disturbing.
Also, the fact that it's Heath's dad looking through it makes it even more fraught with emotion. This clip comes from Heath Ledger: Too Young To Die, a recent German documentary about the actor's life and career. It features extended interviews with Kim, as well as many of Heath's friends and colleagues. It's a fitting tribute to an actor who was only fully appreciated after he died, when he made us all wet our pants by playing a psycho in makeup.