Heath Ledger kept a diary as the Joker where he wrote things that the Joker would find funny, like AIDS and blind babies.
Writer Grant Morrison read a diary that Heath Ledger had written while he prepared to play the Joker on The Dark Knight. Turns out that Morrison's writings had a lot of influence on the way that Ledger portrayed the Joker.
Morrison had written a list of things that the Joker found funny on a comic called The Clown at Midnight. It was prose, as opposed to the typical comic book format, so it wasn't that widely read. Except for Heath Ledger.
Morrison commented on how the diary had a list of things that would make the Joker laugh: Blind babies, AIDS, seriously injured doctors, geniuses with brain damage, landmines, brunch and sombreros. That list was almost word for word what Morrison had put down on page in The Clown at Midnight.