Toni Collette was pregnant with her second child while shooting the show's final season. By the end of the season, she was five months pregnant.
In season 1, episode 11, Tara checks into a mental hospital. It is the same hospital set used in the TV series Scrubs (2001-2010).
Buck is the only alter who is left-handed. All of the others (T, Alice, Shoshana, and of course Tara) are right-handed.
Though all alters appear continuously throughout the series, in season 2, "T" only appears once.
On set, Toni Collette once breastfed her three-month-old daughter, fully dressed as her male identity Buck.
Charmaine was originally not supposed to be a series regular. Rosemarie DeWitt said in an interview that she originally auditioned only for a guest spot.
Portia Doubleday was originally cast as Kate in the pilot episode but was replaced by Brie Larson, when the pilot was picked up to be a series.
Toni Collette and Eddie Izzard worked together once before, in the film Velvet Goldmine (1998).
Brie Larson and Keir Gilchrist previously worked together in the movie Just Peck (2009).
Craig Wright recommended Seth Gabel for the role of Zach after working with him on Dirty Sexy Money (2007).
Brie Larson and Rosemarie Dewitt also worked in Digging for Fire (2015) though their characters never cross paths in the movie
Brie Larson and Keir Gilchrist would both end up working with Anna Boden and Ryan Fleck. Larson starred in Captain Marvel (2019) and Gilchrist starred in It's Kind of a Funny Story (2010).
Toni Collette plays Tara, the older sister of Rosemarie DeWitt's Charmaine. In reality it is Dewitt who is one year older than Collette.
In one episode when Tara (Toni Collette) finds her daughter Kate (Brie Larson) making out with a man in a swimming pool, she says that the Olsen Twins are in town. The twins' other sister, Elizabeth Olsen would later play as Scarlet Witch/Wanda Maximoff alongside Larson (Carol Danvers/Captain Marvel) in the Marvel Cinematic Universe.