The dramatic effects seen in stage hypnosis are mainly due to factors other than hypnosis itself - you wouldn't get people to behave the same way in other situations. The usual setup is:
1/ Volunteers from the audience are not going to be shrinking violets. Thus you have people who are prepared to be make a complete show of themselves, literally.
2/ Suggestibility tests e.g. hand clasp ("your hands are glued together" etc). This ensures only the most hypnotisible volunteers get used. The number of volunteers is further whittled down by upping the ridiculousness-level of the tasks - some will blossom, others will melt back into the audience.
3/ We all know how we are supposed to behave when hypnotised: under the hypnotist's control. We tend to conform to these 'social scripts'.
4/ Ever been on stage, the centre of attention and everyone loving it? Its a great feeling, and you want to do things that get more laughter & applause. And you don't want to let them down by not performing.
5/ Anything you do, you can blame on the hypnotist i.e. a license for disinhibition.
Hypnosis does play a part, but its really the social factors that make people behave outrageously. Note that people who have ever experienced schizophrenia, manic depression, or epilepsy should not volunteer, as the stress combined the hypnosis can trigger or exacerbate their condition. Indeed stage hypnosis is outlawed in many countries.
