I outline novels pretty thoroughly before beginning work. I’ll write a quick one-line sentence of each chapter, then go back and flesh each out into a paragraph, hitting the details and the high points and whatever information has to be delivered in this chapter. Then I’ll settle in and start writing from chapter 1.
At somewhere between chapter 7 and chapter 9, I will decide that the next three chapters are completely useless. Or I’ll figure out that one important detail that had been missing, and realize I need a digression (and possibly a fistfight, or maybe something involving explosions) at this point in the story. So, like a meticulously prepared driver who gets distracted by a billboard advertising FREE PUPPIES AND DONUTS, I’ll veer off into unfamiliar territory, researching as I go, making up twists that may or may not make sense later on. At some point, usually a few chapters later, I’ll come back to the outline, realize that I’ve missed half the important plot points, and staple them on at random points in the remaining chapters.
This means that once the first draft is done, it needs a major revision to even make sense. But those digressions often end up being my favorite parts. And no matter what, the general arc of the story and the ending remain the same. It’s just how I get there that changes.