Heath Ledger as The Joker in The Dark Knight. Photo/Supplied

The diary Heath Ledger kept while preparing for his acclaimed and Oscar-winning portrayal of The Joker in The Dark Knight has been unveiled.

The late actor's journal was revealed by his father Kim Ledger in a German documentary series called Too Young To Die - footage of which was posted to Reddit over the weekend, and widely circulated online.

In the video footage, Kim narrates as he flips through the pages of the journal, which contains hand-written notes, drawings, magazine clippings and photographs of Malcolm McDowell as Alex in Stanley Kubrick's adaptation of A Clockwork Orange.

"This is the Joker's diary," Ledger's dad shares in the clip. "In order to inhabit his character, he [Heath] locked himself up in a hotel room for weeks."

Kim adds, "He would do that. He liked to dive into his characters, but this time he really took it up a notch."

Kim also points to a page highlighting The Dark Knight's hospital scene where The Joker confronts Harvey Dent (Aaron Eckhart) while dressed 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 says. "He looked pretty funny like that. He looked pretty funny in the film, too."

He adds: "Before the end of the shooting he wrote 'bye bye' on the back of the page," admitting, "It was hard to see this."

Per The Hollywood Reporter, the episode featuring Ledger's diary had an original air date of June 28, 2012.

In 2007, Ledger spoke to Empire magazine about his preparation for the part.

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"I definitely feared it," he said of taking on the role in the Christopher Nolan-directed movie.

"Although anything that makes me afraid I guess excites me at the same time. I don't know if I was fearless, but I certainly had to put on a brave face and believe that I have something up my sleeve. Something different..."

He added, "It's a combination of reading all the comic books I could that were relevant to the script and then just closing my eyes and meditating on it.

"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."

The Australian actor, who won a posthumous Oscar for his portrayal of The Joker, died of an accidental overdose of a combination of prescription medications in January 2008. He was 28.

Watch the video below, which is dubbed over in French (a rough translation appears in the description on YouTube):

Bonus: Aaron Eckhart recently chatted with Larry King about working with Ledger on The Dark Knight. Check out what he had to say below:

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