My fellow writers should have something to say on this, though anyone who reads is welcome to comment...
As you know by now, I started the first book of my new firefighter series, Firehouse 343, for NaNoWriMo last month (the book's title is Fire Born). Before NaNo got going, I actually did some pre-planning for the series: I did research (and saved it!) so I'd have quick access to details about firefighting equipment, vehicles, and procedures, and I even planned out my core characters and who their love interests would be. There are also tons of notes on basic plot elements for the first few books in the series.
I believe I've posted before how when I start a story, I only start with a basic idea of what's going to happen. I generally know who is going to be in the book and how they're going to meet, and I have in mind a few points of conflict, action, or other drama to befall the characters. The entire story is never laid out for me from the beginning -- my stories write themselves as I type, which means that anything I had in mind has the potential to change either a little or a lot. Or completely.
This happened more than once in Fire Born. For instance, I didn't know who the villain was until I was more than halfway through the book -- I only knew that I wanted to make it something of a surprise. I knew there would be conflict but not precisely how it would be resolved, and I had no clue how I was going to end the story. I would probably never be able to write an outline like the big publishers want from their writers because I never know what all's going to happen until I write it. Maybe that seems a careless way to write to some of you, but for me that's simply how it works. Besides, I like to write this way because I am never bored by the surprises I come up with -- which might not happen if I planned out every single little detail.
The reason I'm writing this post is because, as I said before, I did some pre-planning that included who the core characters were going to be paired with in their own books. But as I was finishing up the last chapter or so, I started imagining one of the firefighters with another secondary character in this book, rather than who I had originally planned to pair him with. And the funny thing is, the lady I am now going to pair him with wasn't supposed to appear in any other book -- at least, when I first created her, I had only imagined she would appear just this once. She was a plot device, nothing more, and yet as I wrote she evolved into something else. She started to take on a life of her own and I found myself wondering what would happen to her after this story was over.
So yeah, my best-laid plan has already gone astray. I think it's going to be rather fun to see what happens when I get to book 3.