More than five years after his death, Australian actor Heath Ledger's unmatched dedication to his craft was revealed by his father Kim Ledger in a German documentary titled "Too Young To Die."

On Thursday, a short clip was released where the elder Ledger is shown talking about his son's immense engrossment into the role and the dark world of the Batman movie villain Joker.

It was the first time that the father let the public into the kind of meticulous preparation that his son underwent to create the Dark Knight character. An immersion which many believed had contributed to the actor's struggles leading to his early demise.

Heath died in January 2008 in his New York apartment due to accidental overdose.

"This is the diary," Kim says at the beginning of the clip, while he opens a red book filled with scribbled notes, comic book cut-outs, clown photos and some drawings.

Seen beside the actor's posthumous Oscar trophy for best supporting actor, which he won for the Joker role, the diary reveals how Heath had painstakingly worked in detailing the qualities and traits he wanted Joker to possess, exactly how he envisioned him to be.

According to Kim, it was typical of Heath to dive into his characters. But for the Joker role, he pushed it a notch higher. He even went to the extent of locking himself up in a hotel room for weeks just to prepare for the role.

"He galvanized the upcoming character. That was typical of Heath," he said.

"He immersed himself in the upcoming character on every level level."

For the father, the diary is a sad reminder of his son's short life. Heath was only 28 when he was found dead in his apartment in New York in 2008.

While it was ruled that Heath's death was accidental, the diary did prove that the actor wrestled with the painfully perverted role and some heavy personal emotions.

Curiously, the actor wrote a telling note toward the end of the diary. He scribbled in big letters the words "bye bye" at the back of the page where his first photograph as Joker is pasted.

"This is a make-up test which was done eight months before. Before the end of the shooting he wrote ''bye bye'' on the back of the page," the elder Ledger said.

"It was hard to see this."

The movie Dark Knight was released almost six months after Heath's death.

Before he died, he even had an interview with Empire magazine where he openly admitted his "fear" in tackling the Joker role, but at the same time challenged to give it a different flavour.

"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," he said.

"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, and Chris has given me free rein. Which is fun, because there are no real boundaries to what The Joker would say or do. Nothing intimidates him, and everything is a big joke."

Heath had left behind his daughter Matilda with actress Michelle Williams. Both mother and daughter were not with the actor at the time of his death.