Mary Steenburgen, Dan Levy, Alison Brie, Audrey Plaza and Victor Garber and more join the cast of 'Happiest Season"

Pic: ET Canada

As Clea DuVall's Happiest Season — the gay rom-com starring Kristen Stewart and Mackenzie Davis — heads into production in Pittsburgh, the movie has rounded out its supporting cast.

Mary Steenburgen, Victor Garber and Schitt's Creek creator and star Dan Levy are joining the TriStar and Entertainment One project, along with GLOW star Alison Brie and Aubrey Plaza.

Happiest Season follows a woman (Stewart) whose plan to propose to her girlfriend (Davis) while at her family’s annual holiday party is upended when she discovers her partner hasn’t yet come out to her conservative parents (Steenburgen and Garber).

Actress-turned-director DuVall will be making her studio directing debut on the feature, which she co-wrote with Mary Holland.

Holland will also appear in the movie, along with Ana Gasteyer (The Goldbergs), Burl Moseley (Crazy Ex-Girlfriend), Sarayu Blue (Blockers) and Jake McDorman (What We Do in the Shadows).

The project will be produced by Temple Hill's Marty Bowen and Isaac Klausner. Jonathan McCoy and Wyck Godfrey will executive produce.

TriStar will release the film worldwide, with the exception of the United Kingdom and Canada, which will be handled by eOne.

The movie will hit theaters Nov. 20.

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