Characters must start out consistent in their thinking, emotions and behavior at least at the beginning so we can have a sense of how they deal with life. Circumstances might well change the character in one or all three of these areas. Seeing a character acting in keeping with his or her preferred style of thinking and feeling makes him or her more believable to me. I usually create characters from a combination of real people I have know in circumstances found in my stories and projection of how I might act given the circumstances. I sometimes think about what kind of person might be likely to encounter the story circumstances and incorporate characteristics from others I have known.