Joaquin Phoenix lost a lot of weight for his role as the Joker. It was so serious that filming could only be done once, with no opportunity for reshoots. Todd Phillips had to write the script during production.
Joaquin Pheonix's first role in a comic book film. He previously turned down the title role in "Doctor Strange" (2016) as well the chance to replace Edward Norton as the Hulk in "The Avengers" (2012), because he was unwilling to sign onto the multi-picture deal that Marvel Studios was requiring.
The Joker's make-up is very similar to John Wayne Gacy's, a serial killer who would often entertain children while dressed as Pogo the Clown. This make-up style was shunned by working clowns at the time, as they strictly prohibit "sharp" ends in their make-up, as it scares children.
Three of the previous actors who have played the Joker - Jack Nicholson, Heath Ledger, and Jared Leto - are Oscar winners (with Ledger winning for playing The Joker). Joaquin Phoenix has been nominated for an Oscar three times.
Early footage shows Arthur walking by a sign for Amusement Mile, Gotham City's version of Coney Island, which features prominently in the original Killing Joke graphic novel. Joker cocreator Bill Finger was partly inspired by a sign for Steeplechase Park in the real Coney Island which featured a grotesque grinning face.
Due to differences in time zones the first trailer was released on April 4th in Australia which is also the birthday of the late Australian actor Heath Ledger who played the Joker in the Dark Knight (2008)
Batman voice actor Kevin Conroy expressed hope that Joker voice actor Mark Hamill could reprise the role in the film while DC Extended Universe Joker actor Jared Leto wanted to reprise the role as well.
Shailene Woodley was offered the role of Sophie Dumond but turned it down due to scheduling conflicts with Big Little Lies. (2017) Mary-Elizabeth Winstead, Gugu Mbatha-Raw, Dakota Johnson and Aja Naomi King were all considered for the the part. Mary Elizabeth Winstead went on to play Helena Bertinelli/Huntress in Birds of Prey (2020). Zazie Beetz was cast in the role.
The film will reportedly feature a younger actor portraying the Joker and will be separate from the DC Extended Universe and be part of a new film label by DC Comics and Warner Bros. Ironically enough, Joaquin Phoenix is only three years younger than Jared Leto.
Joaquin Phoenix is the fourth Gladiator (2000) actor to star in a DC movie. Russell Crowe was in Man of Steel (2013), Connie Nielsen was in both Wonder Woman movies and Justice League (2017), and Djimon Hounsou is in Aquaman (2018) and Shazam! (2019).
Robert De Niro previously appeared in Heat opposite previous Batman actor Val Kilmer. The opening scene of a bank robbery was also the inspiration for the opening sequence of The Dark Knight. De Niro also turned down a role that went to Jack Nicholson in The Departed.