Heath Ledger's Joker Diary for 'The Dark Knight' Revealed in 'Too Young to Die' Documentary [Watch]

Heath Ledger's cryptic Joker diary from the filming of "The Dark Knight" has emerged in a German documentary.  

A short clip from "Too Young to Die" sees Heath's father, Kim Ledger, leafing through a red notebook, titled "Joker Diary," which the late actor used to prepare for the iconic role in the Christopher Nolan-directed Batman movie.

Kim recounts the life of his son, revealing how intensely the young actor took his roles -- even locking himself away from the world to get into his characters.

"He pretty well locked himself up in a hotel, in his apartment, for a month or so, to sort of galvanize the upcoming character in his own mind," says Kim. "That was typical of Heath on any movie. He would certainly immerse himself in the upcoming character."

The notebook contains sketches and notes on hyenas, The Joker and Alex, the violence-obsessed sociopath from the 1972 classic novel and critically acclaimed film, "A Clockwork Orange."

"I think this was just a whole new level," Kim continues.

Heath's father opens to a page, labeled "Hospital Room," while sharing an anecdote from Heath's childhood:

"The hospital room one was kind of intriguing because his sister, Kate, used to dress him up in a nurse's outfit -- he used to look pretty funny in the nurse's outfit as a kid, and he looked pretty funny in the nurses outfit in the film too."

Heath received a posthumous Oscar for his Joker role. He died in January 2008 of what was ruled as an accidental overdose of prescription drugs by the New York City Medical Examiner, although many speculated whether the intensity of the role drove the 28-year-old to his death.

The journal alone proves the amount of preparation and intensity Ledger took towards his roles.

"There was something about Heath that he would not ever do a part that he could not identify with, and part of his struggle must have been how do you identify with the Joker -- and that must have been pretty chilling and tiring," Ledger's friend, Shakur Kapur, told New York Daily News in 2008.