The Universal-Focus Features' film is being directed by Sam Taylor-Johnson and star Dakota Johnson and Jamie Dornan as Steele and Christian Grey, according to the Hollywood Reporter.
The film's cast also includes Luke Grimes as Christian's brother, Elliot, Victor Rasuk as Anastasia's close friend, Jose and Jennifer Ehle as Anastasia's mother, Carla.
Kate is Steele's best friend and her polar opposite.
Mumford recently starred as the female lead in ABC pilot 'Reckless' opposite Patrick Fugit and directed by Martin Campbell.