Most of the time (if you're like me, anyway), your characters dictate what happens in a story. You may not know what's going to happen next, but if you give your characters a bit of freedom, they may lead the way and figure it out for you. Which is usually great. You want your characters to feel like real people and to do realistic things and make choices that feel natural and not crafted by some supreme overlord...I mean, author. But is it possible to give your characters too much freedom? If you let them get too far away from you, will they do something that makes no sense? And then you're left trying to figure out where to go next or to even use this moment at all.
Think about all of the characters in your story like you're keeping them in a giant pen. You have to constantly make sure the gate is locked and no one gets out. You may let one out once in a while but never let them get too far and eventually you have to herd them back in with the others. And if one gets loose on you, all hell could break loose.
I know it sounds silly, because we created our stories and our characters, so they should be following our ideas, right? Well, not always. If you're really stuck trying to figure out what should happen next, sometimes your characters will figure it out for you. Maybe you just think of every possible scenario and whichever one feels right is the one you end up using. Or maybe an idea just randomly pops into your head and even though you never considered it, it feels perfect. So giving your characters a little freedom can be a good thing if it helps you figure something out.
But there is a very good possibility that your characters could take it too far. Maybe you're too busy paying attention to your main characters and one of the minor ones gets loose and does something silly. I'm dealing with the aftermath of this right now, actually. So I have my main WIP and I've been on the fence about writing a sequel for it for a while, but I did have a second book idea in mind. And I had a character who I imagined being basically just some cardboard cutout in the background. At some point where I wasn't paying attention, he snuck out of the pen.
Suddenly I imagined this character revealing something about himself that made zero sense. He wasn't an important character, so why would I need to know anything about him? If I included this information, the reader would probably be left scratching their head saying, "So?" But the more I thought about it, the more ideas came to me, and eventually I had a complete idea for a third book. Sounds crazy, right? It all happened so fast I didn't even know what to do about it. All because I lost control of one insignificant character. Now I'm stuck not only with another book to right, but with the task of making this character more important than I initially planned. It's not really a bad thing, in the end, but I did not see this coming at all.
So watch out for those stray characters. They may get away from you and do something you never imagined could happen.