Good improv is like a magic act, conjured out of thin air. A handful of people step onto the stage with nothing prepared, and - poof! - an hour of laughs tumbles forth from unpredictable, off-the-cuff scenarios.

No other type of comedy has grown more in the last decade, and now, the burgeoning scene has its first movie. Mike Birbiglia's "Don't Think Twice" lovingly dramatizes the communal but competitive lives of a fictional improv troupe, one whose members bob and weave like featherweight boxers on stage and off it navigate painfully disparate opportunities in show business.

"One night my wife said to me after an improv show: Everyone's kind of equally talented but that person's on 'SNL' and that person's a movie star and that person lives on an air mattress in Queens," Birbiglia says in an interview. "That really knocked me out. In some ways, improv is this great metaphor for life. Life isn't fair."

Birbiglia started in improv and still frequently performs it. But he found his footing in stand-up and one-man, off-Broadway shows that marry comedy and storytelling. After his first directorial effort, "Sleepwalk With Me" (adapted from one of those shows), he became interested in a story about the close-knit friendship of a troupe predicated on supporting each other, even while jealousies fester.

"Yes, and..." - to positively contribute to a fellow performer's ad lib - is the foundational principle of long-form improvisation comedy, as passed down from guru Del Close. It's taught in improv hubs like the UCB Theatre, Second City and the Groundlings, not to mention more than a hundred other theaters that have sprung up nationwide in the last decade.

For his fictional troupe, the Commune, Birbiglia assembled a group of veteran players from such theaters (Keegan-Michael Key, Chris Gethard, Tami Sagher) and actors less practiced in the comic form (Gillian Jacobs, Kate Micucci). Performance scenes, shot fluidly like a fight sequence, were part scripted, part improvised. Before shooting, they gathered for two weeks for an improv boot camp and put on a handful of shows.

"Trial-by-fire improv," Jacobs, the "Community" and "Love" star, calls the training. "It was very intimidating. I was very nervous before we started shooting because not only am I supposed to be OK at it, I'm supposed to be really good."

The top improv theaters and schools have become breading grounds for much higher-paying, higher-profile gigs on network TV and in films. "Saturday Night Live," in particular, looms large. As much as improv is built on positive reinforcement among performers, everyone is also trying to build a solo career.

The story line of "Don't Think Twice" resonated especially for Gethard, who performed in the celebrated improv troupe The Stepfathers along with Bobby Moynihan (now on "SNL") and Zach Woods ("Silicon Valley," ''Ghostbusters"). For Gethard, the film was like reliving 2007-2009, when the break many expected for him didn't come.

"In comedy, there really are only so many jobs. And not everyone gets to have one of those jobs," Gethard says. "In New York, 'SNL' is absolutely one of those gigs that when you're starting out as a comedian in the trenches, everyone kind of has it in the back of their minds. It's right down the street. If you're at UCB, you're 20 blocks away from Rockefeller Center."

The tantalizing nearness of such breaks is something "Don't Think Twice" delves into most movingly through Jacobs' character. An audition for an "SNL"-like show (Seth Barrish plays a not very flattering version of Lorne Michaels) sends her character into a kind of crisis - both out of fear for such a bright spotlight and out of loyalty to the purity of improv.

"I felt different ways at different points shooting it," Jacobs says. "I do like Mike's thought in this film which is that success means different things to different people."

Birbiglia, 38, has his own history in detouring from the most sought-after paths to show-business success. Eight years ago, he thought a CBS pilot was his big ticket. When it didn't get picked up, he instead carved out his own idiosyncratic career.

"At the time, I thought: This is my dream come true. It's all going to happen," he says. "I feel like the fact that I didn't get that sitcom, it's the luckiest thing that ever happened in my life."

Birbiglia has been touring "Don't Think Twice" around the country, and each stop along the way, he invites local improv theaters - the ever-expanding footprint of Improv America - to participate.

"I always say in these improv theaters in Phoenix or Cincinnati or wherever you are: You guys can create the most powerful, best performed, best written, most topical piece of theater tonight in the world. And that shouldn't be overlooked," he says. "You can be performing for 30 people or 50 people or 100 people and, to me, that's more meaningful than starring on a mediocre sitcom that's being half-watched by seven million people."