I think that the default response ought to be to help out. That's what we do as technologists, and our art/science is often arcane and complex in the eyes of layman.

That being said, someone shouldn't abuse the ability to make requests. If this happened repeatedly, maybe even twice, I might say no.

It would depend on my situation, plans, and feelings towards the individual, but in general I don't like being taken advantage of. However if there is some way that I can help others in my job, my inclination is to just help them