Revealed: Heath Ledger's diary which he kept while preparing for Joker

A documentary on the late actor has brought to light Heath's notes written during the making of The Dark Knight

A German documentary -- Too Young To Die -- has brought to light the diary that Heath Ledger had written while preparing for his character Joker for The Dark Knight.

The Oscar-winning actor died at the age of 28 on January 22, 2008 just six months before the release of the film directed by Christopher Nolan. He is said to have suffered from prescription drug intoxication, reports news.com.au.

Many believe that the Australian actor's preparation for the role of the bone-chilling sociopath may have contributed to his depression.

PHOTO: YAHOO

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In the trailer of Too Young To Die, Heath's father -- Kim Ledger -- leafs through his diary that consists of his collection of a joker playing cards, clown makeup pictures and stills from Stanley Kubrick’s A Clockwork Orange.

One of the notes in his diary reads, “Inside. He’s laughing red and black and red and black till there’s nothing left to laugh. Until, almost tenderly, he turns inside out through his mouth.”

PHOTO: ENTERTAINMENTFUSE

"So this is the diary - he pretty well locked himself up in a hotel room for a month or so to galvanise the upcoming character in his own mind," said Kim.

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He continued, "That was typical of Heath on any movie, he would certainly immerse himself in the character and I think this was just a whole new level."

Beside all the scribbles, the word 'Chaos' highlighted in green, clearly stood out in one of the pages.

Heath won an Oscar for the best supporting actor for his unforgettable role as the villain.

Watch the trailer here: