"I think the Joker killed Heath Ledger." So writes attorney Jay Gaskill in his review of The Dark Knight, a film about the superhero Batman. On January 22, 2008, six months before the movie's opening, Heath Ledger, who played the villainous Joker, was found unconscious in his Manhattan apartment. The medical examiner reported that the 28-year-old had died from an accidental overdose of a lethal brew of prescription drugs.

Reviewers lauded Ledger for his "electrifying" performance. Ledger's character is more than a sociopathic master criminal. Reviewers use the language of the supernatural, calling him "demonic" and "diabolical"—"a hound fresh out of hell," "a vivid, compelling picture of … evil," and "like Satan." Michael Caine, who plays Batman's butler Alfred, said that he found Ledger's performance so terrifying and disturbing that he sometimes forgot his lines.

Reportedly, the Joker role had taken a decided toll on the actor's health. For weeks, he was unable to sleep, averaging only two hours a night. He told a New York Times reporter in November 2007 that even after taking two sleeping pills, "I couldn't stop thinking. My body was exhausted, and my mind was still going." He also told a reporter that "the only way that I can act" is to climb inside the skin of the person he was playing. For The Dark Knight, he spent a month alone in a hotel room to work on his character and voice, perfecting an unhinged cackle that sends shivers up the audience's spine. But by immersing himself in the role of the Joker, Ledger might well have gazed too deeply into the abyss.

"Whoever fights monsters should see to it that in the process he does not become a monster. And when you look into an abyss, the abyss also looks into you." This famous but unclear quote by the German philosopher Friedrich Nietzsche has at least one interpretation: if a person gazes too long at evil, it will become a part of him. Did Ledger fall prey to this?