I was watching Succession on HBO the other night, and marveling at the wonderful writing and performing of the various characters.
If you haven't seen the show, pretty much every principal character is a terrible person. But, from my perspective, the actors aren't playing their roles as evil. They're playing them as afraid, and compensating for that fear with evil actions.
Last night, I was reading a scene from an amateur writer where a man and a woman are fighting. The problem with the scene was that the writer wrote them as just fighting. (I know, that doesn't make any sense.) But in real life, when two people fight, their motivation isn't to fight; it's to be heard.
So when you're writing a scene, or acting in one, remember that no one is two-dimensional. Everyone's motivation comes from somewhere deeper, and darker, than what they actually choose to verbalize.