Heath Ledger’s Joker in 2008’s “The Dark Knight” may be the most indelible movie villain of the last decade. Now, five years after the comic-book blockbuster was released and Ledger’s tragic death, fans can get a glimpse of how scrupulous the actor was in preparing for his famous Oscar-winning role. A clip posted on Reddit from the German documentary “Too Young to Die” shows Ledger’s father, Kim Ledger, displaying a diary that his son kept during the filming of “The Dark Knight.” As he says in the clip, “He liked to dive into his characters, but this time he really took it up a notch.” Ledger himself referred to the diary in a 2007 interview with Empire, saying, “I sat around in a hotel room in London for about a month, locked myself away, formed a little diary and experimented with voices -- it was important to try to find a somewhat iconic voice and laugh.