A bird spotted once is a guess. A bird you watch for an hour is knowledge. Same with people.

One sighting gives you a hypothesis and nothing more. Watch for an hour and you get how the thing actually behaves, which is a different category of information altogether, and the gap between the two is where almost every wrong conclusion about a person lives. A first look tells you what somebody is like in that minute, under observation.

So hold your first read as a guess with a question mark on it. Give people three or four separate occasions before you decide, especially the ones who made a bad impression in a situation that would make anybody awkward. The ones who hold up are worth knowing.