The messy middle.
They call it this for a reason, because between the initial action (or inciting incident) and the climax (yet to come) there is a massive chunk of your book that can easily become the hardest part to write, as you're required to fill this space with enough drama and insight to keep your reader hooked.
Throughout the middle, your job is to keep your promise to the reader. That means you're going to follow through with whatever you've set up within the beginning scene(s). The action should be building and conflicts deepening, with everything moving toward a massive collision (and ultimate resolution).
To do this, it's important to keep your "throughline" at the forefront.
Your throughline is your basic plotline, as in: what happens to the main character? How will they be different at the end than they were at the beginning of this story? What will have changed for them?
Any scenes that don't contribute to the throughline should be eliminated. This is the fastest way to tighten your narrative and keep the reader satisfied.
And this will take you from one end of the messy middle to the other.
Be Brilliant!
~Katie~