The surprises that come usually happen at the worst times.
The times when you are tired.
The times where work is farthest from your mind.
You push a bit, and then you push a little more. Suddenly, it dawns on you. A missing piece makes itself known.
A character trait, a plot point, an event that helps the narrative structure make sense.
That is the definition of working while believing in the promise.
The story grows in the only way that it can, words on the page.