This is related to the previous question on plotting. I won’t plow the same ground, but I will mention something else that helps me when I write.
Because of the movement of a story and the expectations of the readers, there are certain scenes that are, in a sense, obligatory. For example, in a crime novel, you will need to have a crime occur in the beginning of your story that is either ruthless, grisly, seemingly impossible to solve, or all three. The detective will then have to visit the scene or at least review the evidence. He’ll look for clues, find something that no one else notices and then follow up on it. He’ll evaluate clues, suspect someone, often have a close encounter or chase scene with the villain, etc…
In a love story, readers need to meet each of the lovers at the beginning of the story and see their lives without each other. Then, there’s the scene where the two meet. They develop an interest in each other, romantic tension deepens and they have to face and overcome obstacles (societal, geographic, socioeconomic, etc…) as they strive to be together or begin a relationship.
Since I know that some of these obligatory scenes will be in my novel, I will sometimes write them out, or write first drafts of them, early on, even though I don’t necessarily know how the characters will get into or out of that situation.
So, by understanding the essential characteristics of every story (tension, believability, escalation, reader empathy, character motivation, causality, etc…) and the the genre expectations of readers, the story develops as you write it. But this is quite different from developing an outline and then fleshing it out.
I can’t imagine anything more boring than writing an outline and then spending six months of my life just filling in the blanks. Eek.