During last Thursday’s IMP rehearsal, there was some hesitation about Hansel. The game was called “Happily Ever After . . . Or Not,” and the improv group of teenage actors were waiting for the narrator, in this case Ashley Girard, to begin the story. Ashley began by introducing the character of Gretel, and Clare Boland rushed into the center of the room. But when Ashley introduced Hansel, the young actors shuffled around the room with no one taking charge.

Carter D'Angelo and Clare Boland. — Ivy Ashe

“Don’t give up!” troupe director Donna Swift said. “Commit to your characters fully. If everyone all runs in to be Hansel, that’s funny.”

The beauty of improv is that spontaneity rules. On any given moment a different Hansel could be born.

The actors then embarked on a journey of “And then what?” which involved Hansel and Gretel going on trial for their witch-killing deeds, recruiting Cinderella’s stepmother as a witness and traveling across a lava pit. “Just like we played in our living room!” said Hansel, now brought to life by Eva Wilson.

Improvisational acting, which doesn’t use a script and requires lightning-fast mental reflexes, is a challenge for even the most skilled thespian. The members of the troupe — Eva Wilson, Ashley Girard, Clare Boland, Carter D’Angelo, Amy Fligor, Aaron Wilson, Alley Ellis, Liam Waite and Della Burke — have been performing since second grade (some began in kindergarten), and performing together since middle school.

Carter D'Angelo. — Ivy Ashe

“These guys have been training for seven to 10 years, and they’ve become masters of their craft,” Ms. Swift said.

This weekend the IMPers will take on their biggest challenge yet, traveling to Chicago to participate in the 16th Annual Chicago Improv Festival. There will be 153 improv troupes at the festival from around the country, with eight countries represented. Only three of these troupes are teen groups, two of which are Apprentice Teams, meaning they train with other groups but don’t get their own show. The IMPers, however, are ready for prime time on their own. For the past three years they have performed at the teen version of the festival, which takes place a week after the main one.

“After the last [teen festival] we felt like we wanted more of a challenge, something newly-created, and so we decided we might as well audition,” Alley Ellis said. The competition was tough — 267 groups auditioned for a spot in the festival lineup. “When we auditioned, we were kind of expecting to get to be an apprentice team,” said Carter. “But we got to be . . . a full troupe, which is very exciting.”

“Chicago’s the improv capital of the world, so it’s sort of a big deal,” Ashley deadpanned.

“I think sometimes these guys . . . because they’re teenagers, people are like, ‘Oh, why would I want to see a teen improv troupe,’ when they’re one of the best teen troupes in the country,” Ms. Swift said. The festival director, who schedules both the teen and the adult shows, had encouraged them to submit an audition tape, she said.

Aaron Wilson. — Ivy Ashe

That tape consisted of a longform improv performance, which Alley described as an “unscripted one-act play.” For the festival’s two 25 minute sets, the IMPers will be drawing from their shortform repertoire of games, like the Happily Ever After exercise. Shortform typically has more audience input, with the crowd offering suggestions for characters or settings.

“It’s like you never know what you’re going to get when you step on stage,” Clare said. “It’s always something new and refreshing.”

“No matter what you do, no matter what character you are, it comes from you, and you’re giving off a message,” Ashley said. “I like how personal it is. You’re not just saying somebody else’s words or being somebody else’s character or thought. It’s all your own.”

“It’s very authentic,” Alley said.

That organic aspect of improv means that one of the more difficult things about performing is not overthinking. It’s easy to get stuck in your own head while deciding on the “best” way to play a scene. But the IMPers have been at this long enough that they all have developed characters to fall back on.

The troupe travels to Chicago this weekend. — Ivy Ashe

“Aaron likes to sometimes make a really big physical choice that’ll spur people on,” Ms. Swift said. “Amy comes more from a place of emotion . . . Della is definitely the commenter. She likes to state the obvious in what’s going on on-stage.”

“We all have a few characters that at any second, you could just pull out,” Clare said.

Over the course of the two-hour rehearsal, character changes were effortless and fluid. There were magic mirrors, giraffes, princesses, in-laws, three little pigs, fairy godmothers and lawyers (to name just a few). There were journeys across deserts, through caves and across tundras. And there were critiques and discussions after each game concluded.

But praise comes in the simplest of forms. Laughter.