Mike Birbiglia’s Don’t Think Twice stands as the best, most revealing film about comedy people and one of the best about artistic collaboration. It’s a boisterous and sensitive work of many facets: tender group portraiture, bang-on media satire, low-key romance, evenhanded inquiry into the ethics of selling out. Above all that it’s an introductory course in improv stage comedy, a work of advocacy for the scene it surveys and — through its warm fascination with improv’s trust-your-group ethos — something like a cult indoctrination.

Birbiglia, the writer-director, has also made it an exhilarating sampler of the art form that is its subject. He opens with a rundown of key rules of improv (say yes to your scene partners’ ideas, put the group before yourself) and then shows us how they shape the performances of the Commune, a troupe at a fictional UCB-like theater in Manhattan.

Of course, those rules, derived from the teachings of Del Close, can apply to life, too, and Birbiglia’s film cannily shows us how the Commune tries — and sometimes fails — to live up to those ideals. But even when his comics break from their troupe, lunging for stardom or stability, Birbiglia demonstrates, through his wide-ranging empathy and his generosity with his cast, that he holds to Close’s philosophy: This is a movie that says “Yes, and” even to the characters who could have been its villains and caricatures.

The film’s heart is shared by Gillian Jacobs and Keegan-Michael Key, who play lovers within the group. Early on, producers of an SNL analogue turn up to watch a Commune performance, and Key’s character, Jack, peacocks for them. Birbiglia’s plot from there smartly charts the fallout: Is Jack wrong to violate the group’s ethos to win his own shot at stardom? Are the other Commune members wrong to hold grudges — or, once he impresses the gnomic Lorne Michaels–like producer, to demand he advocate for them, too?

Jacobs’s Sam, meanwhile, faces down anxiety of the sort that’s usually played for sad laughs in indie comedies. Birbiglia gives Jacobs the freedom to make more of her than that. In the end, when Sam’s started to piece herself together, and to make peace with being broke but creative, Jacobs is giving the richest, grittiest performance of her career, laying bare the character’s heart through her stage work.

Don’t Think Twice

Written and directed by Mike Birbiglia

The Film Arcade

Opens July 22, Landmark Sunshine