People (and characters) are 110%  products of their environments. If you understand the composition of the person, you intuitively know how they would think and act in a given situation. The fun part, as a writer, is in not knowing what each step of the trip from A to B entails until you pen it. You know he has to get there but you seldom know what he will encounter in route. Or at least, I usually don’t.

When these “targets of opportunity” appear, to develop the character or story as you move along, that’s where the good stuff is. No one knows how or why a character reacts in a certain way except the character. As the writer, you are more like a referee at a sporting event. You have no idea which play will be run beforehand, but you know all players must follow the set of rules of their nature and stay in-bounds on the field of their reality. If you allow them to play outside the rules or bounds, the crowds will boo you.

I never know what a dialog will be in advance and usually not the day to day events that just pop up out of no where. The same passage written yesterday or tomorrow would, in all likelihood, be totally different than what the reader sees. Characters often go through the mood changes of the author in other words. lol Jim may have tried to reason with this asshole yesterday, but today he just kicks his with little or no provocation. It all just depends on what mood “Jim” is in the day he was written. If that makes sense at all.