I know it sounds like baptism by fire - and in a way it is - but rehearsals are nothing compared to the real thing. This isn't sketch comedy where everything is laid out for you and there are no surprises. This is improv, which means EVERYTHING is a surprise. Audiences don't always give you the best "gets", or the most appropriate either. You have to learn how to phrase your "asks" so your audience knows exactly what type of "gets" you are looking for. You have to know that it's okay to tell them in your "ask" that you don't want something, or give examples of things you do want. And if you still don't get something that is appropriate or seemingly workable, then you need to be clever and think outside the box to make it work.
Don't misunderstand me; I'm not saying rehearsals are a waste of time. They are absolutely necessary. Rehearsals are where you fine tune characters or try them out for the first time. They are where a group works together to learn or develop games or tweak and adjust them. Rehearsals allow you to become familiar with the elements of scene work so that you can walk onto that stage confident and capable. Basically, rehearsals teach you the games and their gimmicks - they make you competent but not cultivated; ultimately, the shows teach you how to improvise. If you go to a theater and pay a lot of money simply to learn the games, how will you ever prepare yourself for an audience? Like anything learned, it has to be applied to truly be ingrained and understood.
Contrary to popular opinion, performing improv is not about being funny. It's about being creative. And nothing tests your wits and creativity better than a room full of strangers expecting you to improvise a scene using their suggestion.