How often has this happened to you? You're working away on your book. You like it. There are a few snafus, but generally, things are chugging along nicely.
And then, as if out of nowhere, a new idea bursts into your brain and takes a stranglehold on your imagination. Suddenly, what you were working on fades into the background and Hot New Idea is demanding time and attention.
What to do?
Once I had an idea that was just so powerful, I actually jumped ahead a book on a contract and didn't even realize it until I was about half done. Cue panicked call to editor, who was most sympathetic. Actually, she laughed. Because I was so keen, I was ahead of my usual writing schedule so it was still early enough in the process that a change could be made. Another time, I had a character who appeared, fully formed, in Chapter One of a book that was not his. I had to give him a concussion to keep him from taking over.
These are the most extreme examples of Book Hijacking By A New Idea. Usually, I find that if I write down the idea and make notes, that's enough to keep Hot New Idea at bay. I may find that I have more subsidiary ideas and must make more notes, but I don't immediately start writing a scene.
This just happened. I have notes I made on the plane coming home from the conference, because of something an editor said. Picture me shaking a fist at the editor, but while grinning, too, because DANG! Hot New Ideas are just so attractive because at this stage of the game, there are no problems. Everything seems swell. However, I've been in this game long enough to know that eventually, I'll find plenty to work on/fix/complain about with Hot New Idea, too. So I write it down, then get back to my former Hot New Idea. Because it's still Hot, too, in its own way.