“Got your back! Got your back!” a troupe of improv actors chant to each other as they prepare to hit the stage in Mike Birbiglia’s incisive showbiz comedy Don’t Think Twice. Birbiglia, a veteran comedy writer and performer who made his screen directing debut with the stellar rom-com Sleepwalk With Me (2012), frames his second feature as a love letter to the art of improv and its attendant values of trust, openness, and spontaneity. But Don’t Think Twice also exposes the backstage politics of improv and finds beneath the players’ camaraderie an undercurrent of jealousy, rivalry, and professional despair. It has more laughs than any big-studio comedy I’ve seen this year, but it’s dead serious about the difficulty of creating something collectively in a world where everyone’s chasing the spotlight.