In a scene in the teaser trailer, make-upped Joker is going on a stage and dances with Harleen, who is wearing a ball gown. There's a giant shot of a moon in the background. In other words, Harleen is dancing with the devil (Joker) in the pale moonlight.
The Genre Shift to a musical makes sense beyond simply taking advantage of having Lady Gaga when you do a little bit of math and consider Joaquin Phoenix's age (50) to be the age that Arthur is, meaning he would have been born around the 1930s. Between then and the 60s were when many if not most of the classic film musicals would have been released and many of them were Romantic Comediesnote Alternatively, many romantic comedies were musicals which Arthur would have grown up watching or being an aspiring comedian would have at least known about. In the first film, Arthur's hallucinations (his daydream of being on Murray's show and his relationship with Sophie) reflected his loneliness and need for love, validation, and companionship. Now that he's met Harley, his Imagine Spots reflect how his outlook and attitude to that of a love story.
Only problem with this is that Arthur is 32 as confirmed by the film's script and implied in the film itself. Not 50.
