Heath Ledger’s father shared disturbing diary iconic actor kept while preparing for Joker role

Heath Ledger's performance in Christopher Nolan's 2008 blockbuster The Dark Knight is widely considered to be the best portrayal of the Joker.

No offence to Joaquin Pheonix, Mark Hamill or Jack Nicholson of course, but there's a reason Ledger's painted face springs to mind whenever anyone mentions the Clown Prince of Crime.

However, delivering one of the most all-time greatest performances of a comic book character would come at a deep personal cost to Ledger, with the actor pushing himself to the limit in order to prepare for the role.

In the run up to filming, the 10 Things I Hate About You star reportedly locked himself away in a hotel room for weeks while he studied every aspect of the character as well as drawing inspiration from 1971 movie A Clockwork Orange and punk band the Sex Pistols.

Heath Ledger's Joker received universal acclaim (Warner Bros.)

Ledger would famously also keep a diary while becoming the Joker, which detailed his descent into the mind of Gotham's creepiest villain.

The diary would later be released by the star's father, Kim Ledger, in the documentary Too Young to Die: Heath Ledger, which aired eight years after his death at the age of 28.

Produced by German filmmaker Dag Freyer, Ledger Sr could be seen leafing through the journal's pages and confirmed the rumours about his son going into a self-imposed exile was really true.

The pages contained notes and inspirations behind his acting choices, revealing a plethora of terrifying muses including hyenas, clowns, comic strips and Alex DeLarge from Stanley Kubrick's classic film.

Take a look at the journal in a clip from the documentary here:

"He pretty well locked himself up in a hotel room for weeks. He galvanised 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," Kim said.

One particular creepy entry involved the late actor scrawling 'BYE BYE' across one of the pages.

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Ledger would complete filming in October 2007 with the film being in the process of editing when he passed away in January 2008. The actor's death would later be ruled as an accidental drug overdose.

The diary included references to A Clockwork Orange (Broadview Pictures)

And a creepy final 'BYE BYE' sign off (Broadview Pictures)

The Dark Knight premiered to both critical and commercial success in July 2008, earning Ledger a posthumous award for 'Best Supporting Actor' at the Oscars - which his family accepted on his behalf.

He also received a BAFTA and a Golden Globe award, the latter of which was accepted by the film's director, Christopher Nolan.