Heath Ledger's Joker diary unearths secrets behind chilling portrayal in 'Dark Knight'

A German documentary that probes Heath Ledger's death at a young age features a diary that the Australian actor created to prepare for his villainous turn as Joker in Christopher Nolan's The Dark Knight.

There are many theories about Ledger's death at the age of 28 on January 22, 2008 due to prescription drug intoxication and some believe preparing for Joker may have contributed to his depression.

His portrayal of the bone-chilling sociopath earned Ledger a best supporting actor Oscar posthumously and is considered one of the most unforgettable villains ever created in cinema history.

In the documentary Too Young To Die, Ledger's father Kim leafs through the diary which is littered with influences that helped him create the character. "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," Kim said. "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."

In one of the ominous phrases, Ledger writes about Joker, "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."

Ledger also posted several stills from Stanley Kubrick's A Clockwork Orange, which clearly was an inspiration besides joker cards, clowns, hyenas and on one page word 'Chaos' highlighted in green. 

Watch a clip here: