Emma Stone was initially cast as Billie Jean King, but scheduling issues forced her to pass on the role. Brie Larson was ultimately tapped to replace Stone. However, after a few months, Larson dropped out and after Stone's schedule was cleared, she was able to take back the role.
Both leading actors are almost an exact match for the actual ages of the characters they are playing. Billie Jean King was 29 years old and Bobby Riggs was 55 in 1973. At the time of filming Emma Stone was 28 and Steve Carell 54.