Olivia Wilde encouraged Kaitlyn Dever and Beanie Feldstein to spend time living together to really build up a rapport between them. The two ended up being roommates for 10 weeks.
In one of the interviews that are extras on the DVD release, Billie Lourd says that for the scene in which her character, Gigi, dives off the side of a yacht, Lourd was at first quite game to perform the stunt herself. However, the producers told her that it would usually be executed by a professional stunt woman because of the danger involved. When they told Lourd that one of those dangers was that the water she would be diving into was so dirty that she would need to wear a tampon to protect herself from infection, that helped lead Lourd to accede to the idea of using a stunt person. The stunt woman they did bring in for the scene, Heidi Pascoe, was a professional cave-diver.
Olivia Wilde encouraged her cast to rewrite any dialogue for their characters if they felt it to be inauthentic.
Beanie Feldstein initially read the script for another role that was eventually written out. When Olivia Wilde was brought on to direct, she wanted Beanie for the lead role of Molly.
Screenwriter Katie Silberman said that Billie Lourd did such an impressive performance that extra scenes were written specifically for her.