Heath Ledger received a posthumous Oscar for his role in The Dark Knight and now a German documentary is shedding light on how he prepared for the movie, offering a look into a diary he kept to create his version of The Joker.

Kim, Ledger's father, provides the narration for the documentary Too Young to Die. He is shown in a teaser for the documentary talking about how his son prepared for his role as The Joker, leafing through a diary Ledger kept months before the cameras started rolling for the movie.

"This was the diary. He pretty well locked himself up in a hotel, in his apartment, for a month or so, to sort of galvanize the upcoming character in his own mind," said Kim.

Ledger's father added that it was typical of his son to do that on any movie. The Dark Knight may have been one of Heath's bigger roles, but he approached each character he played with the same passion.

Going through the diary, it was easy to understand the great deal of characterization that went into Ledger's role as The Joker. Not only did he make the effort to thoroughly research The Joker in the DC Universe, but he also took inspiration from various elements, compiling photos of hyenas, playing cards and A Clockwork Orange's Alex DeLarge. A reflection of The Joker's state of mind? "Chaos" can also be seen scrawled in all caps and highlighted in green.

One of the most striking attributes of Ledger's Joker diary is that it doesn't look like a Joker diary. While it does say "The Joker" on the cover, the image itself is childish and innocent, possibly hinting at one of the facets he saw of the character.

Ledger himself had talked about the diary when he was making press rounds for the movie, explaining how keeping notes of his thoughts on the character allowed him to experiment with voices as it was important to have an iconic laugh and voice.

"He's just an absolute sociopath ... and Chris has given me free rein," he stated.

At the end of the diary, Ledger scrawled "BYE BYE" over a page, something that Kim admitted was hard to read as it appeared that his son was saying his goodbyes, not just to the character and arduous process Heath had to go through to give The Joker life.

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