I don't even think you should tell the audience you're improvising. It's like an apology in case it's bad : 'we're just making it up' If the improv isn't better than the rehearsed stuff, then you should just rehearse it.
Keith JohnstoneThe improviser has to understand that his first skill lies in releasing his partnerโs imagination.
Keith JohnstoneMy feeling is the moment the theatre is not full you have to do something else. But most people somehow think 'If we did it better, they would come'. No. They don't come.
Keith Johnstone