Here is a relatively common misconception (when we're judging other people's behavior):
No matter the situation, your behavior depends entirely on what kind of person you are.
Well ... that's not such a crazy idea. Put 100 people in the same situation and they won't all act the same. For example, say a bunch of people are taken hostage. Some will cower and cry. Others will plot escape nonstop. A few will try to get friendly with the captors, even if it means tattling on the other hostages. Others will be utterly hostile and completely uncooperative. And some will do whatever it takes to get others out, even at the risk of their own lives. So what would you do? Do you know for sure?
Are you sure?
If you were observing this experiment from the outside, you'd probably be thinking, "What kind of sick, heartless, weak person could do something like that?"
If you were the person with that button beneath your sweaty fingers, you'd probably be thinking, "Why are they making me do this??" Zap.
This is what humans do. When we see someone do something, or when we see something happen to someone (especially something bad), we usually decide that the event occurred because of the person. His/her personality. Genetics. Soul. Whatever. Something inside of that person. You know, that person was lazy. Careless. Stupid. Evil. We go for the "dispositional" explanation rather than the "situational" one. This is so common that it has a name: Fundamental Attribution Error.
But situations have incredible power over us. You've probably heard of the Stanford Prison Experiment? Male college students were randomly assigned to play the roles of guard or prisoner for two weeks in a simulated prison. Except ... the experiment had to be ended after only 6 days.
All the boys in the experiment were chosen because they were normal. Healthy. And within a day or two, they seemed to turn into different people. Some of the things they did--both "guards" and "prisoners"--were quite extreme. Even the lead psychologist on the study, Philip Zimbardo, got into his role as "prison superintendent". Seriously. Even the guy who designed the study was susceptible.
All because of the situation.
If you want a fascinating read on this topic, I suggest The Lucifer Effect.
As you're writing and thinking about character consistency and character development, etc., think about the power of the situation. It's really easy to believe good deeds come from good people and evil deeds come from evil ones, but it's a lot more complicated than that. And isn't that awesome (and a little scary)? It puts villains and heroes in a whole new light and gives you more to play with. Is your villain evil simply because he/she is evil? Is your hero always heroic, never wavering from the good, no matter the situation? For contemporary writers, what kinds of situations can bring out the worst in your characters? What can make them act "uncharacteristically" jerky? Mean? Crazy? Careless? Brash?
So go. Play. Have fun. But your characters have asked me to say this to you: Don't let the power go to your head. You never know what it will make you do ...