Mike Birbiglia's tale of an improv troupe fractured by ambition will win you over.

Don't Think Twice opens with Del Close's rules of improv: say yes, it's all about the group, don't think, and there are no mistakes. It's a handy philosophical guide to The Commune, the fictional New York improv troupe around which Don't Think Twice is based. These six friends are driven, envious of one another's success and occasionally quite selfish, but they're also brilliant performers who shrug off their mistakes and always, always, support each other, on stage and off.

The Commune was founded by Miles (writer/director Mike Birbiglia), a 36-year-old man who lives in a cramped loft and sleeps with his 20-something students. He's seen several of these students go on to greater success while he keeps telling the same story about how he was "inches" from acceptance into Weekend Live, the SNL stand-in that Don't Think Twice skewers with brutal honesty. Miles' most recent student to eclipse him is Keegan-Michael Key's Jack, whose ambition drives him straight into the cast of Weekend Live, a development that his girlfriend (Gillian Jacobs) and fellow Commune colleagues (Chris Gethard, Kate Micucci and Tami Sagher) try, very unsuccessfully, to celebrate. Jack, the most self-involved of The Commune, is thrilled with his hard-won success, but feels conflicted about leaving behind his friends and girlfriend, a dissidence that makes up the bulk of Don't Think Twice's conflict. 

Don't Think Twice is complicated, and all of its characters are complicated. Everyone in The Commune is a good person with selfish tendencies, like most humans. This dichotomy lends a perfect complexity to their performances, a dark/light nuance that deepens every joke. The improv scenes are hilarious and painful, and Birbiglia peopled the cast with UCB and Second City alums. Some of the improv in the film was scripted, and some was actually improvised, but all of it feels completely organic and of the moment. 

The performances are uniformly fantastic. These are very funny actors, but there's so much humanity here, too. Gethard's character is dealing with the hospitalization of his father after a motorcycle accident, and the way the troupe rallies around him while keeping the mood light with gallows humor is really lovely and perfectly weird. Gethard, Micucci, Sagher and Birbiglia all process their envy over Jack's success in different ways - anger, introspection, using it as motivation to spur their own success - but these responses are always funny, a little sad and very real. 

Jacobs is the absolute stand-out of Don't Think Twice. She sees herself growing apart from Jack, despite their best efforts, because they're simply not in the same place anymore. And unlike every other member of The Commune, she doesn't really want to be on Weekend Live. She loves The Commune, loves what improv has done for her life and her self-esteem. As every other member looks at The Commune as a springboard for something greater, to Jacobs, The Commune is that something greater. She gives a poignant performance that is both hilarious and assured, the beating heart of Don't Think Twice that keeps it from just being a movie about six self-seekers and the success that might tear them apart.

Birbiglia's Sleepwalk With Me (which premiered at SXSW 2012) felt like an extended cinematic segment of This American Life, but Don't Think Twice feels like a movie. You might think that a film about a hungry improv troupe would be nothing so much as annoying, and the members of The Commune can feel like a lot, frankly. But the humor and hijinks are met with such genuine, complex feeling that Don't Think Twice will win over the most stubbornly skeptical viewer.