Heath Ledger’s portrayal of the Joker has gone down as one of the best of our time. As pieces of a German documentary on his life resurfaces, we find out why he was able to maintain that role so well.

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In the documentary, the actor’s father; Kim Ledger skims pages of a diary kept by Heath during the filming of The Dark Knight. Between the diary’s pages were playing cards, pictures of clowns, and taped cut-outs from old Batman comics.

Kim reveals his son’s process on getting into character for a film:

“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 really creepy part of the diary? “At the end of shooting, he wrote goodbye on the back of the last page,” said Kim. “It was hard to read that.” Wild.

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SOURCE: COMPLEX