In developing the ‘Joker’ sequel, director Todd Phillips said the team “just found ourselves writing more and more music into it”

“ Joker: Folie à Deux ,” the film’s sequel, now in theaters, sees the return of Phoenix’s Joker, aka Fleck. His bleak incarcerated existence collides with a series of musical numbers — and Lady Gaga as the movie’s Harley Quinn, Harleen “Lee” Quinzel.