Just as there are the character's motivations for the book, the factor is what occurs to trigger an emotional sequence to make the character react and finally act. How do you determine this.
The first is to decide which character will be the one to have the reaction? How is this done?
What does this mean to the character? One doesn't react unless there is some significance to the reacting character. We've all watched when something happens and the way different people react. Some don't do anything but others do. For them the event means something.
Does this action move the story forward, show the character, or provide information to the reader? If not the event will be passed over and probably should be removed from the story. I know I've written scenes and when looking at them later wonder why this was written. That means if really doesn't belong in this story.
Does this mean something to the reader? Will the reader learn something from the event? If not, it doesn't belong in this story but may in another story.