"Improv" refers to improvisational acting which means acting without a script. It's essentially playing make-believe.
You may wonder how adults play make-believe. Is there a bossy person assigning roles, or how does it work?
It works through agreement and addition of information. With each new line of dialogue, one should be accepting what has already been established and adding more information of any kind. One is essentially saying, "Yes, and..." to their scene partners.
When you join us, you'll understand how this plays out. The important thing is understanding that denying or ignoring what your scene partner has said is a sure fire way to make the improv more difficult and less enjoyable.
The Biggest Rule
I learned something while writing this. It's ok in casual conversation to end a sentence with a preposition. I think we're having a pretty casual time here, so I'm going to go ahead and lay out this rule which ends with a preposition:
Be the kind of person other people want to play with.
Be nice. Cooperate. Oftentimes, people have an idea of where they want a scene to go. Due to the collaborative nature of improv, trying to steer the scene doesn't always work out. This is a good thing because every person on stage should feel like they contributed something to what occurred.
This art form isn't about pranking people. The guys on Whose Line Is It Anyway are pretty jocular and have a fun time ribbing one another. They can do that as friends while improvising. As a new person to the group, we encourage that you make friends before you start trying to prank them.
Comedy doesn't always have to "go blue." You may think you're the next Mel Brooks when you keenly assign your female partner the role of a sex worker, but if that suggestion makes her uncomfortable, she'll be less inclined to play with you, because she won't trust your sense of humor. Try to keep it fun for everyone.
Treat your partners like artists and geniuses. If you must manufacture humor it shouldn't be at the expense of your scene partner. Here again, no one will want to play with you, because even though it's make-believe, he or she might believe you're expressing some real animosity that makes them uncomfortable.