Based upon the narrative structure of the first Joker movie starring Joaquin Phoenix, a new theory helps explain why Joker: Folie à Deux will be a musical with Lady Gaga's Harley Quinn. In the first film from director Todd Phillips, Arthur Fleck's disturbing evolution into the Joker is revealed, though not every event that happens to the future Clown Prince of Crime is real. As such, the musical elements and romance promised for Joker 2 could follow a similar path.
Joker: Folie à Deux is set to be a romantic musical with the addition of Lady Gaga as Harley Quinn. As such, it seems the sequel will be inspired by Joker and Harley's romance first seen in Batman: The Animated Series before Harley was brought into the comics' DC Universe. Beginning her origins as a psychiatrist at Arkham Asylum who becomes infatuated with the Joker, it's possible the upcoming sequel could follow a similar origin considering Fleck was institutionalized in Arkham State Hospital at the end of Joker. However, Harley's attraction to Arthur could mirror Fleck's own flawed grasp on reality.
