The film's working title is "Folie a Deux", which means "Madness of Two" in French. This initially led to speculation about Harley Quinn's appearance in the film, which was shortly thereafter confirmed. The name Folie a Deux comes from the 19th century French psychiatrist Charles Lasègue and Jules Falret, also known as Lasègue-Falret syndrome, and it depicts two or more people that they share the same madness or delirium.
Todd Phillips confirmed on June 7, 2022 that the film's initial script is completed, posting a photo of Joaquin Phoenix reading the screenplay. It was also confirmed that co-writer Scott Silver is returning for this film.
This will be the first major DC Comics film to be released under the new "DC Elseworlds" banner. The term was firstly created in 1991 by DC Comics for the stories out of the canon, set in alternate realities.
Like Joaquin Phoenix in both Joker films, Lady Gaga went full method acting for her performance. She insisted that the cast and crew to only call her "Lee" throughout filming. Cinematographer for each Joker film, Lawrence Sher, was quoted saying, "I Never Even Met the Real Lady Gaga on Set, I Didn't Know Her at All."