When Lt. Mike Subelsky transferred from a base in rural Japan to Fort Meade, he looked forward to joining a comedy group here. But, after several months of searching for one, he came up short. So he improvised.

"I started my own," said the 27-year-old Navy officer.

Last January, Subelsky printed fliers and posted messages on Internet forums looking for prospective members.

Eight months later, he had settled on a core group of 13 people, and the Baltimore Improv Group held its first public show. Since then, the group has done roughly one show a month. Its next one will be at 7 p.m. Saturday.

Improv is essentially an unscripted comedy routine. A group acts out a scene based on a suggestion from the audience or plays games. There are no costumes, rarely any props, and the stage is usually sparse. It can feel a bit like a game of charade.

It is hard to pinpoint exactly why the genre is funny. Part of the humor is watching a clever group of people think on their feet and part of it is knowing they might make mistakes. But it often loses its punch in the retelling.

"It is similar to if you have a funny conversation with someone, you had to be there," said Heather Moyer, a 46-year-old journalist and member of the troupe.

As with any live comedy, the good acts cause stomachaches from laughing and the bad troupes have the same effect, but for different reasons.

With stand-up, the individual makes the act. With improv, the group dynamic trumps the talents of any single person.

"When you walk on stage and you don't know what is going to happen, you have to be able to trust the other people," said troupe member Kate Jones, 25, who works in human resources.

Members of the Baltimore Improv Group have this down. They don't talk over one another. The listen and pay close attention to what others are doing.

Most members do have theater backgrounds, and many have participated in improv groups in other cities. They see their range of ages -- from 24 to 48 -- as an asset.

And they work hard. Practice is once a week in the basement of a small farmhouse in Owings Mills. The goal of these sessions is not to learn lines or create shortcuts, but to learn how to listen, how to support and how to play off one another.

In a live improv sketch, "If someone says, 'Help me lift this giant toaster,' you shouldn't say, 'That is not a giant toaster, it is an elephant,' " explained Moyer. Instead, she says, a good member plays off the idea to make it a degree funnier.

Common pitfalls can be being too mean or too raunchy. "I think it takes so much more skill for us to not go cheap," said Jones, who has been in troupes where this has been a problem.

And the group takes a lot of time to focus on the craft outside of rehearsal. Members work on different accents and pay attention to oddities in everyday life that can be incorporated into a sketch. They look for quirky people (sometimes co-workers) they can imitate.

Although it is clearly an intelligent form of comedy, the goal in a sketch is not to think too much. It is to become a character so thoroughly that you are reacting to the situation, not planning what to do next.

"The only thing you can do wrong is not totally commit to the scene," said Subelsky. "If you fail, fail big."

Not a bad philosophy.

The Baltimore Improv Group will perform Saturday at Christ Lutheran Church, 701 S. Charles St. Doors open at 7 p.m., and the show starts at 7:30 p.m. Tickets are $8. Visit www.bigimprov.org or call 410-547-6820.