The New Yorker profile Jessie Buckley, and while there's not a lot re: Ending Things, there's this nugget on what it was like working with CK (SPOILERY):

The movie is, to use the year’s most well-worn adjective, surreal: characters suddenly age thirty years, or break into songs from “Oklahoma!” “The first-ever note I got from Charlie, even in the audition, was ‘This girl is molecular,’ ” Buckley said. “I’m, like, What the hell does ‘molecular’ mean?”

During filming, she and Kaufman would e-mail odd inspirations back and forth: Anne Sexton poems, A.S.M.R. videos. Did she ever figure out what “molecular” meant? “I am a molecule of myself,” she speculated, of her character. “But I’m made up of atoms that Jake has created, which then explode and disintegrate.” Flustered, she blew a raspberry. “I was crap at science.” (Source)