Happiest Season follows Stewart as she prepares to propose to her girlfriend, portrayed by Davis, at her family's annual holiday party.
Things get complicated after Stewart realizes that her girlfriend hasn't come out to her conservative parents, played by Garber and Steenburgen.
Clea DuVall is directing and co-wrote the script with Mary Holland.
Filming started this week in Pittsburgh. Happiest Season is set to hit theaters on Nov. 20.