One of the most memorable lines from The Dark Knight comes from Harvey Dent: "You either die a hero, or you live long enough to see yourself become the villain."

But the Two-Face alter-ego was wrong. Heath Ledger proved you could do both.

By the time Ledger accepted the role as the Joker, he had already become a Hollywood hero. He was the ultimate heartthrob in 10 Things I Hate About You and conveyed his skills as a dramatic actor in The Patriot. He had also secured his first Oscar nomination for his performance in the groundbreaking film Brokeback Mountain. 

Still, the Joker was the role he was destined to play. Not the Joker fans knew from Jack Nicholson's or Cesar Romero's portrayals—but a scarier new villain. Borrowing influence from the drama A Clockwork Orange and the musician Sid Vicious, Ledger and director Christopher Nolan started to form their Joker's identity.

"I'd seen what world it was that I would be playing in. So I knew it was open for a fresh interpretation," Ledger told Joseph McCabe for the book 100 Things Batman Fans Should Know & Do Before They Die. "I also instantly kind of had something up my sleeve, which happened to be exactly what Chris was looking for. We sat down and kind of shared ideas, and they were the same. So, we just went with it."