Joaquin Phoenix Arthur Fleck / Joker Lady Gaga Harleen Quinzel / Harley Quinn Brendan Gleeson Arkham Asylum Guard Catherine Keener Zazie Beetz Sophie Dumond Steve Coogan Harry Lawtey Leigh Gill Gary Ken Leung Jacob Lofland Sharon Washington Social Worker Troy Fromin E Ward Inmate (Troy Larkins) Bill Smitrovich Judge Herman Rothwax