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.
Adam Driver had to repeatedly fly back and forth between New York and London between seasons 3 and 4 to film Star Wars: The Force Awakens (2015).
Marnie is based upon Audrey Gelman, Lena Dunham's real-life best friend. As a tip to that, Gelman was cast in the role of Marnie's rival.
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 four titular characters - Hannah, Marnie, Jessa, and Shoshanna - only appear together in twelve scenes over the entire series.