Execution
During a span of six weeks, Heath Ledger secluded himself in a London hotel room, compiling a character diary and experimenting with voices. "It's a combination of reading all the comic books I could that were relevant to the script and then just closing my eyes and meditating on it", he said about his process. The diary contains photos, news clippings, various handwritten thoughts and words, and portions of dialogue from the film. 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. Furthermore, it contains a list of things the Joker would find funny, such as AIDS, landmines, and geniuses suffering brain damage. It was revealed that Ledger had read Grant Morrison's The Clown at Midnight (Batman #663) and based the list upon the Batman writer's prose.
Ledger highlighted the importance of finding an iconic voice and laugh for the character, relating the voice as "the key to the demented killer". Nolan explained Ledger's early and "peculiar" ambition for the voice of the character, saying that the actor had studied the way ventriloquist dummies talk. The filmmaker also acknowledged that the voice performance was based on the Alexander technique.
Ledger developed the Joker's voice and mannerisms slowly over time and during camera tests. "Don't act, just read it", Nolan had told Ledger for a test screening. In hair and makeup tests, Ledger would start exploring the movements of the character. While test recording without sound, he shared his take on the Joker's voice and physicality, and "in that way he sort of sneaked upon it".
Ledger developed the physical appearance of the character, being "very involved" with the painting of his face, says prosthetic supervisor Conor O'Sullivan. O'Sullivan acknowledged how Ledger, Nolan, and makeup artist John Caglione all gravitated towards a Francis Bacon painting Nolan was referring to. 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.
Nolan noted, "We gave a Francis Bacon spin to [his face]. This corruption, this decay in the texture of the look itself. It's grubby. You can almost imagine what he smells like." Costume designer Lindy Hemming picked inspiration for the "chaotic" look from such countercultural pop culture artists as Iggy Pop, Johnny Rotten, and Sid Vicious. She gave the image for the Joker of someone who is "very sweaty" and who "probably doesn't have a proper home". She tried to present a backstory for the character "that he really doesn't look after himself".
Application of Ledger's makeup was done with the actor scrunching special facial expressions. Caglione called the application work "a dance". This technique created facial textures for white paint. As Ledger closed his eyes tight, Caglione put on the black makeup. Then, water was sprayed over the eyes and the actor would squeeze his eyes and shake his head to create imperfections in the makeup.
To get in character for filming, Ledger kept his Joker diary with him on set and frequently referred to it. Between takes, Ledger would stay in costume and makeup just being himself. The actor would fool around, skateboarding while in his Joker costume on set, and smoking cigarettes. John Caglione described Ledger as helping others around to relax, never letting "the intense nature of the roles overwhelm him".
The first sequence shot was the film's IMAX opening, the "prologue". 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.
The interrogation scene between Batman and the Joker was the first scene shot with Ledger really showing the full performance altogether. The director and his leading actors all liked the idea of shooting the key scene early on. During rehearsals, the actors kept things loose and improvisational, saving for the actual shoot. Christian Bale, who portrayed Batman, confirmed that Ledger did not perform the Joker's voice during rehearsals, waiting to get in character when the cameras rolled. Nolan later acknowledged the scene to be his favorite in the film, saying "I had never seen anybody sell a punch the way Heath was able to do with Christian."
Ledger was allowed to shoot and direct the videos the Joker sends out as warnings. Each take Ledger made was different from the last. Nolan was impressed enough with the first video shoot that he chose not to be present when Ledger filmed the scene with a kidnapped reporter portrayed by Anthony Michael Hall.
Ledger always showed up early on set. The first thing he would do, according to Caglione, was to give bear hugs to cast and crew members around the set. "And no matter how banged-up or bruised Heath was after a long day, after we'd take off the last drop of makeup, he'd just hug everybody in the trailer before he left". At the end of shooting, on his Joker diary's final page, Ledger wrote "BYE BYE".
On January 22, 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. "It was tremendously emotional, right when he passed, having to go back in and look at him every day [during editing]", Nolan recalled. "But the truth is, I feel very lucky to have something productive to do, to have a performance that he was very, very proud of, and that he had entrusted to me to finish". All of Ledger's scenes appear as he completed them in the filming; in editing the film, Nolan added no digital effects to alter Ledger's actual performance posthumously. Nolan dedicated the film in part to Ledger's memory.
Ledger's death affected the marketing campaign for The Dark Knight and both the production and marketing of Terry Gilliam's 2009 fantasy film The Imaginarium of Doctor Parnassus; both Nolan and Gilliam celebrated and paid tribute to Ledger's work in these films. During production of The Dark Knight, Michael Jai White's Gambol was meant to survive his confrontation with Ledger's Joker, which would have resulted in the Joker giving Gambol a Glasgow smile of his own and enabling the filmmakers to reuse Gambol in the future, with White commenting that Gambol was supposed to have a bigger role in further sequels, returning to Gotham City and trying to take it over. After Ledger's passing, Nolan cut Gambol's later scenes and edited the confrontation to suggest but not actually show his death, something White did not find out until seeing the premiere.
Writer David S. Goyer's original intent for the Batman Begins sequels in 2005 involved the Joker being apprehended by Batman with the aid of Commissioner Gordon and Harvey Dent in the second film and the Joker scarring Dent during his trial in the third film. However, most aspects of Goyer's film treatments were absorbed into The Dark Knight. According to Ledger's sister Kate, Ledger was planning to reprise his role as the Joker for another film, a notion supported by Aaron Eckhart, who portrayed Dent, and recalled that Ledger had planned to return in a sequel. Nolan ultimately decided that the Joker would not return in The Dark Knight Rises and dispelled rumors that he would use unused footage from The Dark Knight to bring Ledger back.