There is something in all of us that makes us instinctively want to explain the world around us in terms of people's essential attributes or character: he's a _____ _____.
The mistake we make in thinking of character as something unified and all-encompassing is very similar to a kind of blind spot in the way we process information. When it comes to interpreting other people's behavior, human beings invariably make the mistake of overestimating the importance of fundamental character traits and underestimating the importance of the situation and context.
We do this because we are more attuned to personal cues than contextual cues. Yet, character is more like a bundle of habits and tendencies and interests, loosely bound together and dependent, at certain times, on circumstance and context.
The reason that most of us seem to have a consistent character is that most of us are really good at controlling our environment.
Image yourself in a hostile environment for a sustained period of time and consider how your character might change.