Joker: Folie à Deux,” the sequel to the “Joker,” will be released later this year. The film explores the relationship between Joaquin Phoenix’s the Joker and Lady Gaga’s Harley Quinn, a character that’s being introduced to this universe. Quinn has become a famous character over the past year, largely due to Margot Robbie’s influence, who became the first person to play the role in live action.