Based on the DC Comics supervillain of the same name, he is depicted as a psychopathic criminal mastermind with a warped, sadistic sense of humor who defines himself by his conflict with the vigilante Batman.

In the film, the Joker tests how far Batman will go to save Gotham City from descending into chaos by targeting the Caped Crusader's allies, including police lieutenant James Gordon and district attorney Harvey Dent.

Ledger's portrayal of the Joker as an avatar of anarchy and chaos was specifically influenced by the graphic novels Batman: The Killing Joke and Arkham Asylum: A Serious House on Serious Earth.

He wears the character's traditional purple and green color palette, while his disfigured clown-like appearance is the result of smeared makeup that covers the facial scars of a Glasgow smile (rather than bleached white skin from falling into a tank of chemical waste, like in the source material).

Paintings by Francis Bacon, the character of Alex in Anthony Burgess' novel A Clockwork Orange and Stanley Kubrick's film adaptation of the same name, and various punk rock musicians served as additional inspirations for Ledger's performance.

When the film was released in July 2008, six months after Ledger died from an accidental prescription drug overdose, his performance as the Joker received widespread acclaim and earned numerous posthumous accolades, including the Academy Award for Best Supporting Actor.

[10] The 2005 fantasy film The Brothers Grimm marked a turning point for the actor as director Terry Gilliam gave Ledger (and his co-star, Matt Damon) opportunities to create characters they had not been given before.

[15] Steve Alexander, Ledger's agent, said the actor had a "pay-or-play" deal on The Dark Knight, "so he felt free to do whatever he wanted to do as the Joker, no matter how crazy."

According to The Imaginarium of Doctor Parnassus cinematographer Nicola Pecorini, Ledger had talked with him about Johnny Depp's off-kilter portrayal of Jack Sparrow in Pirates of the Caribbean: The Curse of the Black Pearl in relation to The Dark Knight, aiming to make a performance that would be "so far out he'd be fired".

Christopher Hooton, writing for The Independent, said that the 'Joker journal' had several stills from Stanley Kubrick's film A Clockwork Orange, joker cards, photos of hyenas, unhinged clown makeup and the word "chaos" highlighted in green.

Ledger also got to choose the Joker's weapon among different rubber knives, and he worked closely with costume designer Lindy Hemming on deciding the look for the character.

As the Joker wears a mask through the scene with minimal dialogue, Nolan set the prologue first in the schedule because he wanted to put off performance worries, allowing Ledger to enjoy that relief.

[31] On 22 January 2008, after he had completed filming The Dark Knight, Ledger died, aged 28, of an accidental prescription drug overdose, leading to intense press attention and memorial tributes.

[46] He embodies themes of chaos, anarchy and obsession, expressing a desire to upset Gotham's social order through crime and his conflict with Batman.

His face is covered by a cracked, smeared and runny layer of white clown makeup, while his sunken eyes are thickly rimmed in black and his teeth are yellowed.

[17] About the Joker's physical appearance, Geoff Boucher wrote for the Los Angeles Times that the character probes the facial scars with his tongue and "walks with shoulders bowed and his chin out and down, like a hyena".

In the opening scene of The Dark Knight, the Joker assembles a gang of masked criminals to rob a mafia-controlled bank and tricks them into killing one another, leaving him to escape with the money as the only survivor.

The Joker interrupts a meeting between mob bosses Sal Maroni (Eric Roberts), Gambol (Michael Jai White) and the Chechen (Ritchie Coster), who are discussing a plan to hide their money from the police.

He makes good on his word and targets public figures involved in the mob trial, killing judge Janet Surillo (Nydia Rodriguez Terracina) and police commissioner Gillian B. Loeb (Colin McFarlane), although Gordon saves Mayor Anthony Garcia (Néstor Carbonell).

The Joker also targets district attorney Harvey Dent (Aaron Eckhart) at a fundraising dinner and throws his girlfriend Rachel Dawes (Maggie Gyllenhaal) out of a window, but Batman rescues her.

Having deduced Batman's feelings for Rachel, the Joker reveals she and Dent were taken by corrupt officers to two different buildings after the police chase, and that they are rigged to explode.

While the police are diverted evacuating hospitals and protecting Reese, the Joker meets with a disillusioned Dent, shifting the blame for Rachel's death away from himself and persuading him to take personal revenge on those he deems responsible.

As panic grips the city, the Joker rigs two evacuation ferries - one with civilian passengers and the other with prisoners - with explosives, threatening to blow them both up unless one group sacrifices the other by midnight.

The Joker is not mentioned in The Dark Knight Rises as Nolan felt it was "inappropriate" to acknowledge "a real-life tragedy" in referencing Ledger's death.

[49] However, the film novelization by Greg Cox makes reference to the character while describing Blackgate Prison: Now that the Dent Act had made it all but impossible to cop an insanity plea, it had replaced Arkham Asylum as the preferred location for imprisoning both convicted and suspected felons.

"[3] In his 2016 book The Caped Crusade: Batman and the Rise of Nerd Culture, NPR contributor Glen Weldon recalled that fans were outraged over the choice of Ledger, due to his past roles in films such as Brokeback Mountain (2005).

[65] Roger Ebert of the Chicago Sun-Times stated that Heath Ledger's portrayal is a "key performance" and pondered whether he would become the first posthumous Academy Award-winning actor since Peter Finch in 1976.

"[68] Mark Lee, writing for The Daily Telegraph, commented that Ledger accomplished "a genuinely unsettling, brilliant nuanced portrait of evil".

[79] On playing N'Jadaka / Erik "Killmonger" Stevens in the Marvel Cinematic Universe (MCU) film Black Panther, Michael B. Jordan expressed his admiration for both Ledger's Joker performance, feeling that Ledger's portrayal, as well as Michael Fassbender's Magneto, motivated an actor to deliver an awesome performance as a comic book movie villain.

[80] Discussing his forthcoming portrayal as Kang the Conqueror in the MCU film Ant-Man and the Wasp: Quantumania in anticipation of his further appearances as the character in Avengers: Doomsday and Avengers: Secret Wars, fellow Marvel actor Jonathan Majors named Ledger's Joker as one of his inspirations to play Kang, having admired his Joker version for how far Ledger was willing to physically go during filming of The Dark Knight and for his character's complexity of good and evil, the latter deeply connecting with Majors due to growing up with a few criminals who displayed moral duality from time to time.