"I believe, whatever doesn't kill you, simply makes you...STRANGER!"
In the 2008 live-action theatrical film THE DARK KNIGHT, The Joker was portrayed by the Heath Ledger.
The Joker in Chris Nolan's THE DARK KNIGHT was influenced by several different versions of the character; including the original Joker in BATMAN #1 (1940) and The Joker in the graphic novel THE KILLING JOKE.
Straying a bit from comic book cannon, TDK incarnation’s appearance isn’t a result of falling into a vat of chemicals. This version of the character has a “Glasgow Smile,” white, red, and black clown makeup, and dyed green hair.
TDK's Joker's origin is never explained. "Nothing," says Commissioner Jim Gordon (Gary Oldman) when asked if the GPD were able to identify The Joker. "No matches on prints, DNA, dental. Clothing is custom, no labels. Nothing in his pockets but knives and lint. No name, no other alias."
The Joker alters how his explanation of how his face was scarred throughout the film (ranging from child abuse by an alcoholic father and self-mutilation), adding even more mystery to the man under the makeup.
When explaining the origin of The Joker, director Chris Nolan said that he felt that villains are more scary when their background and history isn't explained. Thus, The Joker in TDK "just IS," explained Nolan. Nolan also said that he took inspiration from the original comic book version of The Joker whose origin wasn't explained until eleven years after the character's creation in 1940.
Heath Leger himself said that he viewed The Joker as a "psychopathic, mass murdering, schizophrenic clown with zero empathy." Ledger also said that he was inspired by punk rocker Sid Vicious and Malcolm McDowell'a character in the film, A CLOCKWORK ORANGE.
"You wanna know how I got these scars?"
The existence of The Joker in “The Dark Knight Trilogy” was revealed at the very end of BATMAN BEGINS (2005) when Lt. Gordon informs Batman (Christian Bale) of a guy who has committed armed robbery and double homicide. He also has a “taste for the theatrical” just like The Batman as he leaves a joker playing card at the scene of his crimes. Batman promises Gordon that he’ll look into it.
In the storyline of THE DARK KNIGHT, The Joker views himself as nothing more than an “agent of chaos.” Not only does he himself take out all of Gotham’s mobsters and insert himself as the city’s #1 gangster, he manages to destroy Gotham’s “White Knight” and symbol for hope, District Attorney Harvey Dent (Aaron Eckhart) by indirectly turning Dent into the villain Two-Face. All of his actions are done for no other reason than to drive Gotham into anarchy for The Joker’s own amusement.
In regards to his relationship with Batman, The Joker says that the two are destined to fight each other forever as neither will kill the other. Why? Because Batman’s moral code won’t allow him to kill The Joker, and The Joker won’t kill Batman because The Dark Knight is “too much fun.”
Batman vs. The Joker in THE DARK KNIGHT
Heath Ledger posthumously won an "Best Supporting Actor" Oscar for his performance as The Joker in THE DARK KNIGHT in 2009. This was the first time an actor in a comic book/superhero film won an Academy Award.
Out of respect for Ledger, The Joker is not mentioned the the film's sequel, THE DARK KNIGHT RISES (2012). However, in RISES's novelization, it's said that The Joker is "...locked away as Arkham’s sole remaining inmate. Or perhaps he had escaped. Nobody was really sure."
On a personal note, I was very proud that Warner Bros. and Chris Nolan chose BOF to debut the final - and best - TDK poster featuring The Joker which you see below (click on it to enlarge).