Touching on your question, I feel that it is down to how you introduce yourself and the image you give to your spectators, example, Derren Brown, famous man and everyone knows him, that being said if he walked up to you and said "sleep" chances are, you would whereas if a random person walks up to you and does the same thing, he is most likely to get a "wtf dude?!" type of reaction. Therefor what I do is introduce myself as a image and confidence of a professional hypnotist without actually saying that I am, what this does is that once they think you are a professional, you create trust and they will most likely to comply with the situation as well as the "ooh! ooh! hypnotize me!" situation. In my work I've never experienced anyone who's been afraid, 99% of the time they're fascinated, which can be a big help too.