The film's subtitle "Folie à Deux" 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 à Deux" comes from the 19th century French psychiatrists Charles Lasègue and Jules Falret. The term was coined to refer to two or more people that share the same madness or delusion. It is also known as Lasègue-Falret syndrome.
During the opening "Me and My Shadow" cartoon sequence one of the crowd outside the theatre has the "Everything Must Go" sign that Arthur Fleck had stolen from him in the first Joker movie.
According to insiders, the success of Joker (2019) and the studio's call for a successor allowed director Todd Phillips to have nearly full artistic control over this sequel. This reportedly got to the point where no one could get through to him: he called it "a Warner movie" and distanced himself from DC Studios as much as possible (the DC logo appears only at the end of the movie), ignored the creative input of DC Studios heads James Gunn and Peter Safran and would only talk to Warner Bros. CEOs Michael De Luca and Pamela Abdy. Phillips also resisted attempts by Warner Bros. Discovery CEO David Zaslav's to move filming from Los Angeles to London (which would have reduced costs by 20%), and refused to have the film test-screened, making the Venice Film Festival the first time that an audience saw the finished film (where it garnered mostly negative reviews). Although Gunn voiced his support for the film online, both he and Safran did not attend the American premiere.
This was the first major DC Comics film to be released under the new "DC Elseworlds" banner. The term was first created in 1991 by DC Comics for the stories out of the canon, set in alternate realities.