You should never discount the value of a good ephiphany whapping you between the eyes at an opportune moment, so that suddenly you realize THIS is the main motivation of your important antagonist / THAT is the thread that can tie all the far-flung parts of your story together / THERE is the plot twist you need in order to pull off the climactic scene, and so on.
For those of us who write without a detailed outline, epiphanies of that sort may be particularly important. Generally I just trust that The Answer (or at least a Good Answer) to a pressing character or plot issue will present itself to me in the nick of time, if not before. Why, I clearly remember, because it was not that long ago, figuring out how to actually end the third Black Dog book when I was . . . wait for it . . . 120,000 words into the manuscript.
Generally speaking, these moments of sudden realization are suffused with a sense of inevitability the moment they occur to you. (Or at least to me.) It seems remarkable you didn’t have that exact detail in mind from the beginning, and ideally when you finish your first draft, it will read as though you did.
I bring this up because you never know what might spark such an epiphany, but most recently for me it was this post by Janet Reid, in particular this passage:
[I]f your character doesn’t have to change, move, decide, risk something in the first 50 pages, it’s often a pass from me.
THAT’S IT! I cried, because I had been struggling with chapter three or so of one of my current Works In Progress for ages. (That struggle is one major element that has led to my switching back and forth from one WIP to another all summer long.) I knew it was a problem with passivity or inaction or both, but somehow framing it as The protagonist needs to make a decision by page fifty did the job. I immediately revised chapter three (again, sigh), this time finally putting an important decision into the protagonist’s hands, where it obviously had belonged all alone.
Sometimes it’s something that is just that obvious, and was just that obvious all along, yet somehow you didn’t see it.
I strongly suspect that you can’t get an epiphany of that sort to happen when it would be convenient, ie before you write the dratted chapter in the first place. I think you have to have the problem in the back of your mind for long enough that the solution is ready to crash down on you in an unmissable OF COURSE DUH moment. Then any little thing can bring it down. But for me, first I have to write it wrong before I can do it over right.