It seems to hit most often just when I am relaxing. I'm all snuggled into my blankets and just about to drift off to sleep. Suddenly, a new story or a new scene or plot point bursts into action in my mind. You want to just fall asleep letting the scene play out, but you know what will happen -- You'll never remember it in the morning.
You're happily writing away spending quality time with your characters when suddenly a new set of characters arrive. These new characters with a whole different story to tell just don't want to wait their turn.
It's a great thing, don't get me wrong. I'd rather have more ideas than I can handle than not enough. My trusty notebook that I keep handy for all sorts of random note taking has gotten a workout. Scenes pop into my head that have nothing to do with my current work in progress and are written down so that I don't forget them.
The most annoying part, is these ideas don't pop up fully formed, of course. They just tease. A few characters chime in with only pieces of their story. I know it is just to tempt me to focus on them and delve into their story even when I'm in the middle of working with my other characters.
You'd think they could learn some manners, but I guess when your story needs to be told, it just needs to be told. One of these days, I hope I can get character voice mail or something. They could just leave me a message and I could get back to them when I had more time.
Think it might happen? Guess for now I'll keep my notebook at the ready. It's worked so far.