When you play the Sims 3 game, you build a new Sim (character) with a number of traits. These vary from being good to evil, neat or sloppy, to social to shy. These Sims will then behave true to their nature. Neat Sims clean up after themselves, slobs eat off their plate with enthusiasm, and then leave the plate to attract flies. Evil character cause fights, while the good sim develops friends.
This ties into storywriting. I’ve read stories where the character ends up doing something completely ‘out of character’ with who and what they are. Perhaps its to add drama, but as a reader, its tough for me to buy into what the writer dictates to the start of the story as the traits they’ve set into place.
Our traits are not inborn, but often nurtured through Life’s experiences. People who lived through trauma carry that with them, whether they’ve grown to better people, or it hangs on them like deadweight. Such experiences mold and shape a character to the ‘person’ they are- don’t have them switch too quickly.
An evil character, for instance, works from their weaknesses or some trait they perceive to work for them. They don’t just become good for the sake of being good. They must endure a pivotal event that somehow shakes the core of their believes (or shakes them from that weakness) to rise above. Such morals often shift in the other directions as well.
Consider the traits of your characters, and be true to that character. They will thank you for it.