It wasn't a secret that Heath Ledger went to extreme lengths to prepare for his role as the Joker in The Dark Knight, going so far as to put together a diary from the perspective of the movie's deranged antagonist. In this one minute clip, Ledger's father, Kim, gives us a brief glimpse at the contents of that journal.
The clip, which looks to have been taken from the German documentary series "Too Young To Die", shows Kim Ledger paging through the diary and providing some context to a few of the entries. Unfortunately, it's hard to make out what Ledger is saying due to the translated narration however, this is easily remedied by switching on the English subtitles.
Here's a snippet from the interview (the transcription courtesy of The Film Stage):
The hospital scene is interesting because when he was a kid, his sister Kate liked to dress him up as a nurse. He was really funny like that ... This is a make-up test which was done eight months before. Before the end of the shooting he wrote "bye bye" on the back of the page. It was hard to see this.
It's a unique insight into the mind of the actor -- one can only wander what else is contained inside the diary and if we'll ever see its contents in full.