A scene from a German documentary about the life of Heath Ledger shows off the diary that Ledger kept for a month during his preparation for the role as the Joker. In the video, Heath's dad, Kim Ledger, flips through the diary and talks about his son's approach to acting. 

"He pretty well locked himself up in a hotel in his apartment for a month or so, to sort of galvanized the upcoming character in his own mind," said Kim. "That was typical of Heath on any movie. He would certainly immerse himself in the upcoming character. I think this was just a whole new level."

The diary has playing cards, photos of clowns, cut-outs from old Batman comics taped into it. There are also a lot of images of the character Alex from A Clockwork Orange

One of the weirdest, saddest parts of the diary is that Heath made a note in it eight months later after the movie had wrapped. In what looks like silver Sharpie, he wrote, "8 months ago, wrapped now!" Then, on the last page of the diary, the words "BYE BYE" also appear in the same style. 

"At the end of shooting, he wrote goodbye on the back of the last page," said Kim. "It was hard to read that."

Watch the clip below.