Good ideas are shiny. When you try to sleep, they twinkle with possibilities behind your closed eyes, and if you let yourself look at them, sleep is impossible and excitement rushes through you as if tomorrow were Christmas.
It's an indefinable quality. With bad ideas, the more you think about them, the more problems you notice. With good ideas, any problem is outshined by the sheer uniqueness and potential of the idea. Some ideas take a while to get shiny. They seem dull and normal until you turn them, looking at them from another angle and they suddenly catch the light and blind you and you wonder how you didn't see it the first time.
To me, EVERY point in a story is a time when it could branch off into a different direction. Iggy could be in the middle of a jail cell, wondering if he should try to escape or if he should wait for his trial, when an earthquake hits. Suddenly Iggy's situation is completely different.
When I'm at a fork in the road the thing I try to remember is "where is my destination?" If I go this way, will it take me closer to the emotional story I want to tell, or further from it? There are thousands of ways your story could work. Sometimes you have to make a decision and say "I'm going to introduce an albino chimpanzee right now" and stick with it.
Making a decision is hard enough, but it's even harder when I tell myself "I can always go back and change it if I need to." I've learned from bad experience that "keeping options open" doesn't work for writing a book, because there are literally endless possibilities when it comes to the decisions we writers have to make with our stories.
With every option that's left open, for every decision that's only halfheartedly made, we begin juggling multiple stories in our minds. There's the story I'm writing, and the story that I could have been writing. It's like listening to two different radio stations at the same time. "If this doesn't work, I can just go back and go the other way" makes it harder to stick with past decisions when they, inevitably, get difficult later on.