As artists paint on canvas, writers have to paint pictures with words. But I often write in layers as painters will do touch ups on their pictures to make the colors blend better. The same goes in adding detail to a scene or a character by returning to read and revise previous text.
The other day, I went back several chapters to revise. Here and there, I added this, condensed that, but slowly the scenes became clearer. By the time I finished, I had added two more pages to the novel while fleshing it out.
While I never write out an outline beforehand, I often find that the first run through is like the skeleton of the story. As I go back, time after time, more clarity and direction unfold until the skeleton becomes its own person with a unique identity. Exactly how this occurs is still a mystery for me, but that's my process and it works. Believe me, there are days when I don't think it will work, but I keep plugging away.