The two were inspired by 1970s character studies and the works of Martin Scorsese (who was attached to produce early in production); the graphic novel Batman: The Killing Joke (1988) served as the basis for the film's premise, but Phillips and Silver otherwise did not look to comics for inspiration. Phoenix became attached to the project in February 2018 and was cast that July, while the majority of the cast signed on by August. Principal photography began in September 2018, taking place in New York City, Jersey City, and Newark, and concluded the following December.