The Middle is Where Story Happens
A good beginning sets up expectations and a solid ending resolves the story. In the beginning, your reader enters the story world, meets the protagonist, and encounters a trigger that sets up the story. In the end, your protagonist challenges the villain and resolves the story question, leaving your reader with a sense of satisfaction. Between those two story points lies the great, vast middle.
That vast middle is where your story happens. You deepen the story enlarging on the promise at the beginning. The middle expands your story. You delve into the theme, create subplots that echo and support the main storyline, and enrich the understanding of your protagonist. You do this by creating a line of conflicts that put more at stake as the story unfolds.
The middle is where you deliver on the story promise you made in the beginning.
For the sake of clarity, I’ll use the four-act structure: beginning, before the midpoint, after the midpoint, and climax/resolution, so we all understand where we are in the story’s progress.