7. Cailee Spaeny As Ruth Summerspring
Let's make something clear right off the bat; none of the performances in this film are anywhere close to being less than great. They're just differing levels of great and the one who draws the shortest straw of the bunch is Cailee Spaeny.
Spaeny is an up-and-comer who's about to be so much more, with upcoming roles in films like Adam McKay's Vice and Miriam Leder's On the Basis of Sex. In this film, she plays Ruth Summerspring and of the seven lead characters, she gets the least amount of screentime or development. Part of this stems from the fact that Ruth is a character who is a beta to not one, but two different alphas in the narrative.
She is Emily Summerspring's little sister, and Emily frequently acts paternally towards her, attempting to save her. Simultaneously, Billy Lee is the leader of the cult she is a part of, and she spends the entirety of the film obeying his every whim. The dependency of these two relationships (especially Billy Lee's), limits the amount of growth the character can go through.
Spaeny is admirably dedicated to the role, bringing an off-the-wall energy and ferocity to Ruth, but she simply doesn't get as much material to work with as her co-stars.