Looking back on comic book movies from the last 25 years or so, few performances stand out like Heath Ledger as The Joker in the 2008’s The Dark Knight. His twisted, manic take on the quintessential Batman villain redefined the character for Nolan’s dark, grounded film and the approach resonated with audiences. When Ledger died months before the film was released, his death was the most-reported entertainment story of the year, and became inseparable from both his portrayal of The Joker as well as the film on a whole.

Ledger was first approached by Nolan to play the title character in 2005’s Batman Begins. Ledger replied politely that he’d “never do this kind of film,” and the role would eventually go to Christian Bale. The following year, when discussing the sequel, Nolan announced that Ledger would be cast as The Joker, and that he was “talented but fearless,” promising audiences that “watching Heath Ledger’s interpretation of this iconic character taking on Christian Bale’s Batman is going to be incredible.”

Regardless of Nolan’s assurances, when Ledger’s casting was announced, it was subject to the scrutiny of Bat-fans all over the internet. Of course there were similar reactions to Michael Keaton’s casting as Batman in the late 1980s, as well as Ben Affleck when it was announced he’d don the cowl for Dawn of Justice back in 2013. While Keaton and Affleck each won over many of the doubters, only Ledger managed to undo virtually every skeptical fan’s doubts.

During a Q&A at the Film Society of Lincoln Center in 2012, Nolan said that he never had to approach Ledger for the role, because, in his own words, “Heath chose me.”

He just was determined to do it. He just had a vision for something, and the way he termed it to me at the time was, he really didn’t like to work too much. He liked to do a character and then stop working then let enough time go by. He wanted to be hungry for it. And when he came to me, he was clearly in that state: Very hungry. He was ready to do something like that and just own it — which is what he did.

Ledger was no stranger to Method acting, and he took his commitment to the role seriously. Being cast before the script was written, Nolan explained that Ledger had “a very long time to obsess about it, think about what he was going to do, to really figure it out.” In addition, Nolan had Ledger read Anthony Burgess’ A Clockwork Orange and study the paintings of Francis Bacon. Then, months before shooting began, Ledger isolated himself, writing and collaging images to help him get inside the character’s head. When Ledger spoke to Empire Online from the set of The Dark Knight in 2007, he explained his process.

I sat around in a hotel room in London for about a month, locked myself away, formed a little diary and experimented with voices — it was important to try to find a somewhat iconic voice and laugh. I ended up landing more in the realm of a psychopath — someone with very little to no conscience towards his acts.

When production began, Ledger would be on set every day in full costume and makeup — but he’d only be in character when filming. Make-up artist John Caglione told Movie Geeks United in 2012 that Ledger would skateboard around, goof off, smoke cigarettes, and give out bear hugs at the beginning and the end of every workday. When it was time for him to work, however, he had his Joker diary close by to bring the character out at will.

As Ledger threw himself into the film, his sleep began to suffer. He told The New York Times when filming in London in 2007 that he “probably slept an average of two hours a night. I couldn’t stop thinking. My body was exhausted, and my mind was still going.” The article makes note of his manic behavior and compulsive ticks, his general restlessness, as well as the Ambien he began taking to help him sleep.

His Joker diary also comes up, observed by the interviewer when it was laying on a counter in the kitchen of his rental house, which Ledger sheepishly explained is there to help give him the character’s backstory.