This is an interesting topic!
I have to say, yes. I'll write a couple lines of dialogue before working on an episode, and then for some reason, my MCs change what they say when I finally give them speech bubbles. And sometimes do something unexpected/funny. Least that's my interpretation.
Frankly, as the true Mother Hen of my universe, Friar totally disregards what I'm trying to tell him, to stop blindly following and start leading. My upcoming Saturday episode was not set in stone, but it sort of evolved by itself from a stray idea that hit me last summer. Never intended to put through my characters through what you're about to see, but they sort of fit their roles and let the story go in an interesting tangent that promotes character growth, or let's them shine for a bit.
Suppose that's the opposite of what you've asked, but I know my characters too darn well to let them take over my plots so easily. The only one that I worry about is Bucky, for he is a free-er spirit than the rest. But we'll see how that goes. It's still a little early, but I feel the spotlight will be hitting him more and more.
If you want some vague advice, try to flesh out your characters/predict how they'll react in situations, planned and unplanned. It requires A LOT of thinking ahead and imagination(been doing it for a few years even though the main plot points took only 6 months to write). But it helps to know every possible action/reaction of your MCs so as to not write the story too far off course (and if it does go a little astray, find ways to work in reactions/events that'll return your chars to their previous state, or close. Unless you hit like a climatic/big-change event that changes your MCs permanently. I know it's common good storytelling practice to change your characters through plot but not everything needs to change, and not so quickly especially if your story unfolds a little slowly like mine does. Friar will still be his arrogant self, despite what he's about to go through, but it'll only set him up for bigger things to come later down the line...
On a related note, put a training leash on those MCs of yours, lol!
Edit: I make a comic...just realized this was in writing category...oops...