Mr-Grue - My take is that participants in a stage show have allowed themselves to do what they do. they can always blame the hypnotist for it afterwards!

Some participants do, however, get genuinely swept up in the clever wordplay and trickery taking place (which includes a lot of psychological and physiological effects) and will genuinely believe they have been 'taken over' but the reality is they haven't.
Beeno - just because a hypnotist bamboozles someone so much that they give up and comply does not mean there is anything going on re most peoples conception as to what hypnosis is. Really this debate will boil down to semantics regarding definitions. Yes, you can use psychological and physiological stimulus and 'trickery' to persuade someone to think or act in a way you desire BUT they are under no more of a mystic influence or different state of mind than the specs you will manipulate through clever patter and deceptive distractions etc when performing magic. Hypnosis does take this several notches further but in essence is no different. For example, you have probably been in a shopping centre/mall and experienced that dazed feeling induced by the heat and constant hubbub etc (sorry to the compulsive shoppers out there - you may not have felt this in your state of heightened excitement

). When you have been in that state of mind you are probably more likely to just go along with the flow than resist. Amplify that several fold and add some methods of inducing that sort of feeling and you get close to what hypnosis is. A common analogy is when you are reading a book or watching TV and someone can enter the room, even speak to you, and you are so absorbed by something else that you don't recollect their being there. Nobody is saying nothing happens in somebodies brain just that it is not as mystical as made out. Actually, with stage performances there is a high likelihood that nothing has happened at all other than the spec deciding for whatever reason to play along.
Its a confusing thing to convey in written form. If you get the opportunity go and book a session with a hypnotherapist (just for a relaxation session if you want) or volunteer to participate in a stage show. You will find there are many similarities and a few differences in how things are done (different hypnotists use different methods of course and there are some who do stage acts which are 100% fake just to add to the confusion) but experience the 'state' for yourself and you'll probably come out of it wondering what all the fuss was about. Sorry, if my attempts at explaining are not up to scratch but, baseline, I do persuade people to do things but it is them who opt to do it and there is no mystic stuff in it just as in any other 'magic' or the salesman persuading you to buy a certain product.
What makes hypnosis so interesting (to me as a psychotherapist rather than a performer) is how even many of those practicing it don't realise how they are creating the effect. We could get into very deep philosophical conversation as well as discussing neurological processes which can be measured but doing so only confuses the matter further as there ARE processes happening (even when someone decides to just act & play along) but the topic will invariably return to semantics and (fortuitously for me) a case of whether you decide to believe I and others can externally effect your brain or not.
If there were a way to use hypnotism the way the majority perceive it all hypnotists would be fantastically wealthy and being befriended by politicians and salesmen along with those magicians who can make money appear from nowhere and fly through the air.