Monsters, toys, cars—Pixar's protagonists are often nonhuman. In the studio's latest movie, though, the main characters aren't even physical beings. The central players of Inside Out, hitting theaters today, are emotions—Joy, Fear, Sadness, Anger, and Disgust—inhabiting a young girl's mind and brought to life by a murderer's row of voice talent like Amy Poehler and Bill Hader. But while casting was a breeze, picking the emotions wasn't. (Schadenfreude was an early contender.) Turns out there's no consensus on how many emotions there actually are—so director Pete Docter and his team turned to an expert they'd worked with before.

Early in the movie's five-year production, Docter invited psychologist Paul Ekman to brief the crew on the nature of emotions. According to Ekman, who has had a long and sometimes controversial career, there are seven emotions with universal facial signals—those that ended up in the movie, plus contempt and surprise. For story purposes, Docter needed simplicity; five, he says, was “just enough for dissension and entertainment.”

Ekman applauds the movie for teaching kids and parents alike about emotion—that understanding, he says, allows us to “choose what we feel, rather than being controlled by our emotions.” Then again, if we could choose what we feel, we wouldn't cry so much during Pixar movies.