There is a point in any story when no matter what you had planned, it seems like the characters decide they know better.
Sometimes this is good.
Sometimes this is bad.
If one of them gets the bit between their teeth and runs in the wrong direction, it’s not good. Sure, it adds words to your count, but if it takes the story into Never Never Land, where it should not be, that isn’t such a good thing. It could destroy the tale you wanted to tell and no amount of editing after NANO is going to cure that.
I’ve had secondary characters attempt to take over a story. One in particular wanted to be the hero when he was supposed to be the sidekick. Mentally, we had a meeting. I promised him his own book where he COULD be the hero. I had no idea what the story would be, but he was happy as a lark and settled into his sub-starring role for the manuscript I was writing. Oddly enough, because I already knew him really well by the end of that original story, when it came to giving him his own, everything fell together faster. I also ended up liking his story better. That could be because I got to spend more time with him.
What happens far more often is that a character who is holding down the hero or heroine’s role suddenly grabs the wheel and gives it a yank doing something I did not plan. Yes, it creates complications, but in dealing with those complications, the story gained something it hadn’t possessed in my original plan…lose though it was. I am a Pantser, after all. Well, 75% one with Plotter picking up the other 25%. Still, the character causing my concept to catch its toe and trip did bring me to a complete stop on the story for a while. Once I realized he knew exactly what he was doing, and let him have his way, things rolled along nicely.
The point here is, when a character attempts to take control or tosses surprise moves at you, don’t freak. Deal. Either make a deal with the character or accept that the “WHAT!!!” you gasped is an opportunity to improve the story.