Heath Ledger's Joker was more than just a role. The actor took his role seriously and even kept a Joker diary while preparing to play the famous villain that appeared in The Dark Knight, directed by Christopher Nolan.Ledger died on January 22, 2008, reportedly from an accidental overdose. During an interview with Empire, talking about the famous role, Ledger revealed that he locked himself away for a month for the role and even started a diary where he wrote ideas about the voice, laugh, and way of thinking.The diary appeared in the documentary Heath Ledger: Too Young to Die and was shown by his father, Kim Ledger. Let's look at what Heath Ledger kept in his Joker diary, including Batman comics, handwritten notes, and more.Heath Ledger's Joker diary in Too Young to Die reveals his Joker preparationA scene from The Dark Knight (Image via YouTube/@HBO Max)Batman comics and character researchThe pages in the Joker Diary by Heath Ledger included Batman comic clippings and other material that helped him understand the character before bringing his own version of the Joker to life. But that was not all. Ledger also wrote down his own thoughts about the character. To Heath Ledger, the character, as reported in Empire, was:"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"The diary also included a few lines from Nolan's film, including the Joker's "It's simple. Kill the Batman" line, which Ledger wrote in uneven capital letters, and the famous line "If you're good at something, never do it for free," which he wrote in the diary as "Like my mother always told me, when you're good at something, don't do it for free."The laugh and the handwritten notes View this post on Instagram Instagram PostAnother page that stands out in the diary is the picture of a hyena. It has often been linked to the strange laugh Ledger created for his character, Joker.One of the strangest pages in the diary was titled "THINGS THAT MAKE ME LAUGH." Ledger listed many things under it, including blind babies, land mines, AIDS, beloved pets in road accidents, statistics, pencil cases, BRUNCH!, and the periodic table of the elements.Speaking about the Joker's personality and his laugh, Ledger said in a 2007 interview with Empire that he spent about a month alone in a London hotel room while preparing for the role. During that time, he kept a small diary and tried out different voices and laughs because he wanted to create something people would remember. He said he eventually saw the Joker as a psychopath with almost no guilt about what he did.The final entry in the Joker diaryThen comes the page, the last diary entry by Heath Ledger, that has caught the most attention of his fans. Ledger wrote, two words which were "BYE BYE." Ledger died months before The Dark Knight was released in theaters, which made the simple final entry that stated "BYE BYE" even more noticeable and left many of his fans wondering about the meaning behind those words.The diary had many other images too, including photos from Stanley Kubrick's A Clockwork Orange and photos of Joker cards, along with examples of wild clown makeup. One line on the page says "8 months ago, wrapped now!"In a 2007 interview with Empire, Heath Ledger said he was nervous about playing the Joker because fans expected a lot from the character. But he also said that the fear excited him and pushed him to try something different.Also Read: Tracker Season 4 Episode 1 cast: Meet the actors who brought Safe Harbor to life