I love it when that happens.
For me, writing a book has that same moment. Intellectually I know I'm going to write a story that comes floating into my head, but emotionally, it often doesn't click until I'm in the middle of actually writing it down.
I'm at work on the sequel to KIDNAP.org, which got nice attention from people in the book industry as well as readers. I knew I wanted it to be a three-story arc, and with my editor's suggestion, I figured out what the 2nd and 3rd books would deal with. I started writing, and it went well. Robin and the gang face two threats, a very powerful target and a sneaky man from her past. I got through a rough draft and one edit, and as I went, things got clearer and better.
And then I started the second edit. All sorts of things happened in my head, and suddenly the story came alive. I can hear Robin talking. I can see Cam's pained expression when they tell him he has to wear dress shoes for a whole evening. I can feel Tom's panic when he realizes his friends are locked inside a murderer's estate.
It isn't finished--not even close. But that moment has come where I fall into the story headfirst, and I won't be able to rest until it's done...even though I already know how they're all going to end up.