The character of Shoshanna wasn't supposed to be recurring, but creator Lena Dunham liked Zosia Mamet's performance so much that kept her on the show as a regular, because they saw potential for exploring the character thanks to Mamet's talent.
Jemima Kirke had to be convinced by Lena Dunham to take the role of Jessa, because Kirke didn't (and doesn't) have a long-term interest in acting, compared to her focus on her thriving career as a painter and artist.
The celebrity connections of actors and members of the creative team of Girls (2012) continue to expand: the cast includes the daughters of David Mamet, Brian Williams and Simon Kirke, the composer is Sean Penn's brother and three episodes have been directed by the son of publisher Martin Peretz. In addition, actress Audrey Gelman, who's been featured in two episodes, is a former press aide to Hillary Clinton who now serves as the press officer for Manhattan's Borough President.
Dakota Johnson auditioned for this show. Unfortunately, didn't get the part but got offered to audition for 5-Year Engagement by Judd Apatow. Later, becoming widely known through her talents in Fifty Shades of Grey, Black Mass and many more.
The trivia item below may give away important plot points.
Christopher Abbott (Charlie) left the show before the third season started filming because of creative differences with creator Lena Dunham, resulting in the relationship of Charlie and Marnie ending abruptly off-screen between the second and third season.