Ten years ago this week, the 28-year-old actor shocked the world with his sudden death.
His final performance as The Joker was released posthumously and won him an Oscar for Best Supporting Actor.
Considered his best role by many, his director Nolan has reflected on what it was like working with Ledger in creating the Joker.
Speaking with BBC Radio 1, the filmmaker said: “He would sort of give me hints about what he was going to do. We would talk about it a bit. And I would try and be an audience for him and sort of engage with him, what he was doing.”
Nolan continued: “But a lot of it was about unpredictability, and I think he wanted to play his cards close to the chest.
“He would very gradually reveal to me the ‘voice’ and the way he was going to do things – but not in one go, like, ‘Here’s the Joker.’
“We watched him sort of develop it, with the wardrobe and the makeup, and I kind of got to be a part of that creative process, which was great fun, but on-set, there were always moments like that clapping or things he would do with his voice.
“His voice was so unpredictable. He created this bizarre pitch.”
Nolan added: “I’ve seen a lot of people try and imitate it since. But we never quite knew if he was going to go high or if he was going to go low.
“You never knew what that guy was going to do, and that’s what was terrifying about him.”