According to an interview with The Hollywood Reporter, Jonathan Nolan shares that he and the studio didn't get why Heath Ledger was cast as the joker.
Jonathan, as in brother to Christopher Nolan, the director of 'The Dark Knight,' says in the interview that “No one got it, the studio didn’t get it. Everyone was coming at Chris [Nolan] and saying, ‘we don’t see it’.”
He goes on to say “And the fan community was … we were f—ing pilloried for it... It was a disaster. This was the worst casting decision ever made. Chris just hunkered down and stuck to his guns and kept moving along."
It ended up being the best casting decision for the movie, considering that not only did Heath make what's called one of the best Jokers of all time, but according to NME, Ledger's sister shared that Heath had every intention of working with Christopher Nolan on another Batman film.
“He was so proud of what he had done in Batman,” she was quoted saying, “And I know he had plans for another Batman. He loved working with Chris Nolan and Christian Bale and Gary Oldman. He just had the best time ever doing that film."
Heath Ledger was one of the only actors to win an Oscar for Best Supporting Actor posthumously (joining Ralph Richardson of 'Greystoke: The Legend of Tarzan, Lord of the Apes') and it was very well deserved.
VIDEO: All Of Heath Ledger's Scenes As The Joker In 'The Dark Knight'
Such an incredible performance, There will never be a better joker.