William Poundstone in Biggest Secrets examined hypnotist/comedians, people like Pat Collins who supposedly can convince women that their breasts expanded and men that their penis fell off. Sounds medically sound to me.
Some people believe all hypnotism is fake. Many others believe in clinical hypnosis but think the stuff on stage is a joke. Poundstone looked into manuals and professional literature for stage hypnotists (I would have never thought such stuff existed) and found that while the authors seemed to believe what they were doing was real, they repeatedly reveal that, at the very least, it isn't reliable.
Many hypnotists use their friends as volunteers in the audience ("I've never seen this gentleman before in my life") but many look for extroverts and people who don't want to disappoint or let others down. (Covington noted that while he doubted a stage hypnotist was legit, he wouldn't want to go on stage and ruin his act. Apparently, this is what keeps them in business.)
Usually hypnotists start their acts by "mass hypnotizing" the entire audience. Some people refuse to play along at all. They won't wind up on stage. Others respond half-heartedly. The hypnotist looks for those who enthusiastically follow directions with a "look at me!" attitude. (Ralph Nader would be perfect.)
Poundstone quotes Ormond McGills' Encyclopedia of Genuine Stage Hypnotism: "Whenever a laugh comes in your show for no apparent reason, be on your guard that someone is faking, and spot that trouble-maker as soon as possible." In other words, look out for the smart ass who goes on stage and flips you off when your back is turned. If stage hypnotism really was genuine, this wouldn't seem to be possible.
McGill also advises about using chloroform to "greatly assist you in hypnotizing the most stubborn cases." Other hypnotists use itching powder or other chemicals. Hypnotist Bill Larson favored the "bulldog method" in which you squeeze the subjects' carotid arteries to cause them to go limp. This method is favored by rapists with medical know-how.
There are other tricks involving lighters and sledge hammers that are also used by cult leaders to fool their subjects into thinking they have mystical powers. I guess I'd rather have someone use their knowledge for a fake hypnosis act than to start a money-grubbing cult.