How much of our initial "knowledge" about another person is fantasy?

We have this rush to judgment built into us as a survival trait. So we often think we know something about a person based on how they look or talk or act. We think we know something because of what they say they do.

When it’s “love at first sight,” what goes on in a person’s head? What do they think they know about the other person? Why do they believe it’s true? How often could it be true?

Even when you date, do first impressions bear out? How often? How often do you think a person is one way, and they turn out to be quite another? When do our fantasies about other people turn into real knowledge?

I am calling it fantasy, but I think it would also be fair to call it a “hypothesis” or an “intuition.” Is calling it a fantasy unfair? Is there a functional difference between fantasies, hypotheses and intuitions? Is there much difference between the fantasies generated when meeting a person in real life compared to meeting someone online?