Character Motivations & Reactions
All characters must be motivated either internally or externally, to behave in a certain manner. Their reactions must come from the feelings they would experience due to the motivating stimulus. What is significant to one character may not be significant to another, depending on their background and current circumstances.
For example, a farmer who is facing a large crop loss due to a drought would react happily to the prediction of heavy rains, whereas a person who recently lost their home in a flood might feel despair, or panic about the prospect of another rainfall.
When describing all of this you'll start with the motivating stimulus, and then what comes next must be in chronological order.
1) What exactly has happened to change the current state of affairs for your character?
2) What does your character now feel?
3) What action does he take?
4) What does he say?
For example:
Farmer John hears a radio report that two inches of much needed rain is in the forecast.
That's the motivating stimulus.
Relief floods his body, his shoulders finally release tension.
This is how he feels.
Then he lets out a happy cheer which his wife hears from across the yard.
This is how he acts.
John says, "Did you hear that? Finally some rain!"
That is what he says.
If you do this out of order, it starts to feel unnatural or disjointed from how people behave in the real world.
Additionally, you have to create characters so in-depth that you could predict how they might behave in response to something as small as an impending storm, or a lost earring, or a surprise visitor at their doorstep.
You will spend days making up entire backstories for each character. I give them names of course, but then I create entire family trees for them, jobs, financial status, health concerns, physical appearance, relationship history, personality traits, etc., so that when I bring them to life in the story I know what kinds of things would set them off, and how they might react.
This is what makes a story feel believable. It is also incredibly important to establish the story world. You can never assume that your reader has been where you are, and you must feed them sensory details. What can your character see, hear, touch, taste or smell?