You write a scene and it's about encroaching darkness. And when you rewrite, you realize that logically, you have to move some action around. And you described it pretty well the first time, but when you move it, you realize that it reflects atmospherically the wrong thing. If the movement is from infra-red to ultra-violet in tone, suddenly you have slapped gobs of paint over the middle. Action is right, but the the center of it is unaccountably dark, then goes back to lighter.
That's sort of it. Also, that diminuendo thing. Closing the doors, drawing the shades, making sure the windows are latched. Have I spent the right balance of time with everyone? Does one scene feed into the other, now that I've chopped out the center of the rainbow, flipped it, and taped it vertically to the bottom 1/3rd? No, of course not.