Sometimes, and this hasn't happened to me recently, I begin thinking of a person -- a character, and they come to some sort of definition in my mind. Their likes and dislikes, habits, appearance, the sort of place they live in, and so on. But this all happens pretty quickly, with the result being a more or less fully-formed human instantiation that exists as a hierarchy of sharply-defined traits. I remembered one such character recently because I encountered a character very much like him in an actual novel. It gave me a feeling not unlike the one that you get when you read about an invention that you think you pioneered in isolation, and didn't bother to follow through on, bursting onto the scene and changing peoples' lives, or at least making boatloads of cash for the inventor.