"You're not a failure, Bill. You're in The Commune," so declares Samantha, the undaunted peacekeeper of a flailing improv group. Don't Think Twice is based on Mike Birbiglia's firsthand experience as a member of a comedy troupe similar to The Groundlings. He wrote, co-stars in, and nimbly directed this story of a troubled ensemble. But the camera keeps returning to Gillian Jacobs' performance as Sam, bringing her emotional life into focus in each consecutive scene. Without seeming effortful, she provides the moral compass for a close group of 30-somethings who act like overgrown adolescents. They are committed to little besides their improv mantra — "Yes, and..." — and to having each other's backs onstage. The problem is they have yet to make it, until one of them does. Jack (Keegan-Michael Key) lands an audition and then a job at Weekend Live, a fictional version of SNL, throwing off The Commune's equilibrium. (Adding to the trouble, he is also Sam's boyfriend.) Birbiglia finds subtle ways of comparing Jacobs with Katharine Hepburn and Gena Rowlands, but it's Greta Garbo whom she summons to the screen as the director films her profile in a medieval light, casting her face in mystery. Jacobs projects something new here, a remote aspect with a shadowy half-life. Don't Think Twice portrays the distress of love as simply a distraction from Samantha's companions and their good work together — the rare achievement of a collective soul.