If you have seen people do crazy things at a hypnosis show, it is natural to wonder how much control the hypnotist has. It is important to remember that people who volunteer for stage hypnosis have an expectation about what participating will entail. The hypnotist selects people who seem uninhibited, and sends the reluctant back to their seats.
Context and motivation determine what makes sense as a suggestion. You will reject things that don't make sense. It would be difficult if not impossible to get someone to cluck like a chicken in my office, just as it would be difficult to suggest that the star of a hypnosis show quit smoking.
Hypnosis is a consensual state. I am literally incapable of making you do anything you're not comfortable doing. If I asked you under hypnosis to tell me your ATM PIN number, you would just say "no" and laugh at me (in hypnosis it is completely normal to talk, laugh, scratch an itch, or shift to get more comfortable in your chair).