The best investment I made in my writing career was purchasing Robert McKee's audio book, "Story: Style, Structure, Substance, and the Principles of Screenwriting." I listened to the audio book repetitively and I wrote out the book by hand in my relentless pursuit to screenwriting mastery.
Robert Mckee's explanation of the gap between expectation and result is a story principal that all the master storytellers process. This gap is so important that Mckee said it was the nexus of storytelling:
"A substance of story is the gap that splits open between what a human being expects to happen when he takes an action and what really does happen. As a charge of electricity leaps from pole to pole in a magnet so the spark of life ignites across the gap between the pole of self and the pole of reality. With this flash of energy, we ignite the power of story and move the heart of the audience."
The story gap is energetic, kinetic and emotional. Storytellers have to be conscience of this gap. If you don't master the gap then your stories run the risk of being stagnant, cold, disengaging. Everything that is counter intuitive to the 'flash of energy,' that McKee speaks of. Mckee also said "that if the scene is about what the scene is about then you're in deep shit."
Screenwriters Robert Zemeckis and Bob Gale were very mindful of the story gap when they wrote Back to the Future. You could be on the right track towards writing something as iconic as Back to the Future if you master the art of the gap between expectation and result.
The story gap during the climatic scene of Back to the Future is one of the best demonstrations of Robert McKee's story gap.