I evaluated a recent speech where the speaker had great body language, gestures and eye contact, yet I found it extremely hard to understand exactly what his point was. It turns out that part of the problem was that the demonstration that he did with two audience members turned out terrible. He wanted them to say, “Yeah, this is great, it’s the best drink in the world,” and they ended up saying “No. No.”
I’ve had the same type of problem when using audience members. I tell them to do one thing and they end up doing something else. The solution is obviously to practice ahead of time with your participants. Rehearse the part of the speech where you use your audience and get them to rehearse their part. Don’t just talk about it in vague terms, but have them say explicitly what they are going to say during your speech.