Imagine casting a world-renowned singer in your multi-million dollar movie and then telling her, "Hey, you know that thing that you're amazing at? Can you maybe try sucking at it?" That's exactly what Todd Phillips and Joaquin Phoenix did for the upcoming Joker: Folie Deux. In the sequel, Lady Gaga plays Harleen Quinzel, a woman who falls in love with Arthur Fleck (Phoenix), a.k.a.