Predicting Character Reactions

Have you ever come to a pause while writing, trying to figure out how your character would react to a situation? Humans beings are complicated creatures. They're completely unpredictable. Around some people we can act one way, and around others we can be an entirely different person. Often times, we don't even know ourselves well enough to predict our own reactions. Your character must also have this quality if you want them to be truly realistic. But this creates a quandary. How exactly would you achieve such dichotomous character reactions? How can you predict the unpredictable? Read on.