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 JohnstoneImagination is as effortless as perception, unless we think it might be โwrongโ, which is what our education encourages us to believe.
Keith Johnstone