If your story moves at 100% the entire time--endless drama and action--the reader will likely wear out. It's also unlikely they will be able to keep pace without backstory, or exposition, that explains who these characters are, why they matter, and how they stumbled into the problems they're currently facing.
Exposition, however, does slow down your narrative.
This can be a good thing (it gives us a break from the action) or a bad thing (it can make us put down the book).
Nancy Kress (Beginnings, Middles & Ends) has a Swimming Pool Theory that helps with this:
"The stronger and more forceful your initial kick, the longer you can glide through the water. The stronger and more forceful your opening scene, the less your reader will mind a 'glide' through nondramatized backfill."
The key is balance.
Give us that kick of action, then let us glide. Give another kick, then glide. . . .
Be Brilliant!
~Katie~
