Heath Ledger kept a chilling diary in preparation for his Oscar-winning portrayal of the Joker in The Dark Knight.

The Australian actor died of an accidental overdose of a combination of prescription medications in January 2008.

His journal was revealed by his father Kim Ledger in a German documentary called Too Young To Die. In it are pictures of hyenas, clowns, comic strips and photographs of Malcolm McDowell’s Alex in Stanley Kubrick’s adaptation of A Clockwork Orange.

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“He pretty well locked himself up in a hotel room for weeks,” Kim told the filmmakers.

“He galvanized the upcoming character. That was typical of Heath. He would do that. He liked to dive into his characters, but this time he really took it up a notch.”

In addition, the actor wrote out the famous hospital scene in which he confronts Harvey Dent in a nurse’s outfit.

“The hospital scene is interesting because when he was a kid, his sister Kate liked to dress him up as a nurse,” Kim said. “He looked pretty funny like that. He looked pretty funny in the film, too.”

On the last page, Ledger scrawled “Bye Bye”.

“At the end of shooting, he wrote goodbye on the back of the last page. It was hard to read that,” his father said.

In 2007, Ledger told Empire magazine about his intense preparation for his role in Christopher Nolan’s Batman franchise.

“I sat around in a hotel room in London for about a month, locked myself away, formed a little diary and experimented with voices – it was important to try to find a somewhat iconic voice and laugh. I ended up landing more in the realm of a psychopath – someone with very little to no conscience towards his acts. He’s just an absolute sociopath, a cold-blooded, mass-murdering clown.”