It's not enough to have a great idea that's really interesting. It's not enough to plan carefully, and work out a perfectly precise outline. It's not even enough to create highly engaging characters. You've got to really sink into the story, until it comes so naturally you're practically speaking in an accent to your family members.
Drowning In It
Here's an example of what I mean: my newest book is about a girl who's always lived her life by the ocean. I have not. But in the book, I used quite a few different metaphors and phrases that are ocean-related. But I didn't have to think about using them or struggle to put them in. That's what being in the zone means, I think. When you're really deep into the story, things that fit into it just start coming to you naturally.