Funny thing, I start out with a piece of conversation, or simply a place in the story, and then branch out in all directions from that point. Odd way to do it, but it works for me.
Case in point, my short story about the atheist who spent three days in Heaven got its start from a smart ass quip about what gay Christians do in Heaven, which is pull weeds in the garden.
The scariest part of writing I’ve ever dealt with was when a main character in my novella about global warming and a totalitarian civilization in the future ‘spoke’ to me, telling me why he had to die, and what purpose his sacrifice would serve. That was three years ago. I haven’t touched that work since.