A German documentary, titled Heath Ledger: Too Young To Die, was released in 2012 and chronicled the rise and fall of the iconic actor. A clip from the film, which features Ledger’s father taking a look into his son’s character diary for the Joker, has gone viral over the past few days. 

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“So, this is the diary—he pretty well locked himself up in a hotel room for a month or so to galvanize the upcoming character in his own mind,” explains his father, Keith, (via ConsequenceOfSound). “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.”

Heath Ledger died from an accidental overdose of prescription drugs in January of 2008, just months before the theatrical debut of The Dark Knight.