Heath Ledger's performance in 2008's The Dark Knight was immediately iconic, earning the actor a posthumous Academy Award for what, a decade later, director Christopher Nolan remembers as a "terrifying" portrayal of The Joker.

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Over the years, many actors have stepped into the shoes of Batman's arch-nemesis, from Cesar Romero to Jack Nicholson to Jared Leto, with each bringing something different to the role. However, Ledger's performance as a nameless maniac who wanted to watch the world burn stands out from the rest. On the 10th anniversary of the actor's untimely death, Nolan talked with BBC Radio 1 about working with him on The Dark Knight:

“… 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. 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.’"

While it sounds scary enough to contemplate the lengths Ledger went to nail the part, Nolan revealed the actor's unpredictability added another layer of fear to the performance:

“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. 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.”

Sadly, while Ledger's Joker survived the events of the movie to head on a presumed trip to Arkham Asylum, the actor's life was tragically cut short before the release of The Dark Knight. Nolan has remained coy about where he would have taken the character next (if he would have returned at all). However, the story moved on, and out of respect for Ledger, The Joker's fate was never revisited.

As the first superhero movie to gross more than $1 billion at the box office, The Dark Knight left behind an impressive legacy due to a gripping storyline and complex performances. However, there was a tragic twist to the tale, with Ledger's diary revealing that he had gone perhaps too far into method acting. Nolan reflected on Ledger's posthumous Oscar:

“I took huge pride in having been in any way involved with this great performer and his legacy. He was an extraordinary person and an extraordinary actor. And for him to be recognized in that way, I think, was very meaningful for his family and meaningful, I think, for film history. What he contributed, and he contributed in many different ways to film history, but that it be marked in that way, I was very proud to be a part of it.”

These days, Leto plays an even edgier version of The Joker in the DC Extended Universe, but has divided critics on whether he's right for the part. Perhaps Ledger was just too good as the Clown Prince of Crime, ensuring that any subsequent Joker actors will be measured by the standard of his performance.