The Australian star won a posthumous Oscar for his performance as the creepy criminal mastermind in the superhero movie, having died six months before the film's release after suffering an accidental overdose of prescription drugs in 2008.

It has now emerged Ledger wrote a disturbing journal while he was preparing for the part, and it has been shown off in a documentary about the actor called Too Young To Die.

A trailer for the film shows his father Kim leafing through the diary, showing off Ledger's collection of joker playing cards, clown make-up pictures and stills from Stanley Kubrick's A Clockwork Orange. The journal also features bizarre lists and scrawlings, including a note which reads, "Inside. He's laughing red and black and red and black till there's nothing left to laugh. Until, almost tenderly, he turns inside out through his mouth."

Kim Ledger says of the Joker journal, "So this is the diary - he pretty well locked himself up in a hotel room for a month or so to galvanise the upcoming character in his own mind... That was typical of Heath on any movie, he would certainly immerse himself in the character and I think this was just a whole new level."