Joaquin Phoenix and Lady Gaga in "Joker: Folie à Deux."Warner Bros.The first trailer for "Joker: Folie à Deux" screened at CinemaCon on Tuesday night.Director Todd Phillips said, "Lady Gaga is magic," when asked why she was cast to play Harley Quinn.Watch the trailer below. The movie opens in theaters on October 4.During the Warner Bros.