Retired hypnotist here. Probably more qualified to answer your question than most.

Let's talk about hypnosis first. You've already experienced it many times even if you've never been "hypnotized" by someone like me either in an office or on stage. Have you ever seen a scary movie? Ever gotten excited watching a show when the hero is in danger? You know that they're just actors in front of a camera and there's no real danger at all yet you have allowed yourself to get carried away by the show. Your heart rate might change, you might hold your breath until the danger is passed. You might even jump out of your seat! What's the deal with that? The deal is simple. You've suspended some rational thought processes so that you could be entertained. You're giving something to get something. It's no different than the people on stage. They do what they do because they get something out of it. Some enjoy being part of the show. Others want to believe so they believe. The best ones of all say something like "I didn't have to do it. I just did it to go along." They are right. They did not have to do it but they chose to act because the felt the need to go along with the show. They followed the hypnotist's directions (suggestions) even though they did not have to.

Anyone and everyone is suggestible. If we weren't the advertising business would have tanked long ago. Not everyone is suggestible to the same things and not everyone makes a good subject on stage.

A hypnotist is really someone who knows something about human nature. They are trained to see how people react and communicate. A good hypnotist knows how to communicate on different levels using a number of devices to do so.

I'll leave you with a question. Who is the hypnotist hypnotizing? Is it the volunteer on stage or is it the whole audience?