When I am planning a story, I always have a pretty good idea of how my characters will be feeling in any particular scene. However, I never feel 100% certain until I'm "on the ground" in a scene. This is one of the reasons why I tend to write in linear chronological order - but even if you don't, it's worth taking the time to go through the story in linear fashion to make sure all the motivations connect up to one another.
For me, plotting a character's reaction to something is not a simple matter of stimulus-response. I'd write out my way of thinking through the process like this:
- initial mental state
- initial mental state + emotion inspired by judgment
Characters perceive plot events, judge them and react emotionally, which then causes them to feel a motivation to act in response.
Sometimes, especially in action, this occurs very quickly. I don't have to write out a separate sentence for each step in the process! But before I have a character enter an interaction, I go through in my head how he/she is feeling and why. Emotions concatenate. If we're already feeling tense, our reactions to a particular event will likely be magnified. If we're feeling rattled because of previous events, we may not be able to slow down enough to notice things, or to think through our response to what happens next.
As I write, I find myself thinking through the nature of emotions and motivations. What kind of emotional state might makes a person get so angry they might start throwing things? If a person often reacts in one particular way without thinking, what might be different about their reaction if they decided to do it on purpose? What does a character want to accomplish by their actions? Are they fully in control or on the edge? What behaviors do they engage in to counteract the feeling of being out of control? What could push them so far they might make a decision that hurts them in the long run? Why would they hurt a friend?
A character's actions, and particularly a point of view character's actions, must grow naturally out of his/her reaction to story events. If they don't, the sense of deep connection and plausibility will be lost. If you have a story that alternates point of view, make sure to ask yourself what happened in Character A's story while you were visiting Character B's head. It's important, because when you go back to Character A, that person's reactions won't necessarily have much to do with what was going on with Character B (unless they were in the same scene together). The state of mind with which they enter their next point of view scene will depend on what they were up to "offstage." So it's very good to have figured out what happened offstage! In fact, if I don't know what happened and what state of mind my character is in, I can't start a new scene at all.
By going through all this, I aim to create a sense of mental continuity with each character that runs from one end of a story to the other. If I find the plot requirements are dragging me off that continuity, then I either go back and change the character motivation so it will end up in the right place... or I change the plot.
It's something to think about.