I'm currently working on the sequel. Well, it's more like I'm wandering around, or at least that's how it feels as I wend my way through the middle of the story. I don't do outlines other than the synopsis my editor gets and I generally tend to be pretty vague about the middle of the story at that point. I know the beginning, I know the crisis and the climax and how the story's going to end. But the middle? Uhhhh....stuff happens.
But that's also where the unexpected happens, in no small part because I've kept things vague. That leaves me wiggle room to explore new characters or plot ideas, and to come up with unforeseen elements of the characters' histories.
So while it's a bit of a nail-biter to be wandering through uncharted territory, it's thrilling, too.