First of all why are you asking us..? I for one would ask a layperson this, not other magicians. However, I believe they think about a male man(Not a female. I don't know why they don't think of a female magician, maybe because the assistant is usually a female, either way it's quite sexist.) in a suit and tie with a top hat which he can pull a rabbit out of.
There have only been a few times I have had a laymen say anything to me after telling them I am a magician. They were all co-workers. These where their exact questions. First time - Can you float like the magicians on TV? Second time - Can you do magic like the guys on TV? Most recently - Do you do magic for birthdays? I also juggle, so thats where the last question came from.