Since I don't write my scenes in chronological order, I don't always now how certain details came to be. For instance, there is a girl in my current wip who has a phobia. I know why she has it, I know the cover story she uses to keep people from noticing, and I know how she'll work through it in the end. What I have never known was HOW she came up with this cover story or WHEN exactly she started telling it.
So, in the past few days, I've redone my outline, started revising from the beginning to reflect the changes, and now I'm filling in missing scenes as I go.
Just now, in chapter nine, it came to me. She needed to say something, so she said it. And it makes perfect sense that she would say it there because of the events of the previous scene. If all fits together so perfectly that it's going to be one of those things that readers will think I planned from the start. But honestly, I didn't even know this scene in chapter nine was going to exist until three days ago.
It's like a piece of the puzzle that was there all along but I never even knew it!