I'm pretty sure it's every Pantser's neener-neener on Plotters. I can hear their sniggering as the scene ends. The outline has us storming the castle, yet the characters decided to stowaway on an alien ship. They've jumped timelines, quadrants, and genres.
And yes, sometimes it really is that drastic a change. When a character fully comes alive on the page, the story I thought I was writing can turn out to be a wholly different beast.
Let me tell you, dear readers, I am one of those folks who believes spontaneity is fine...
...as long as it's planned.
For the most part, however, I roll with it. Whatever the characters are up to, I indulge. For a time. As long as they still have the same GMC and can get themselves to the pre-arranged Grand Climax, I will revise the outline and keep going in the direction the characters have determined.
On the other hand, if those naughty little boys and girls are on a tangent, then I am ever the cruel mistress who will drag their asses back to the scene prior to their unfortunate display of free will and force them down the path I had originally plotted.
Mother knows best, after all.
What about personalities that develop contrary to how I envisioned them? This, this happens all the time. They adopt traits and make decisions that completely change their arc. Hell, they occasionally have the gall to inform me that two or three distinct characters I'd originally planned are actually facets of one character. Protagonist down to tertiary character, their growth from concept into multi-faceted person is what keeps the magic of story-writing intact for this Plotter. I typically have three to five Big Moments that I need the characters to hit over the course of the story. As long as they get there, I delight in the surprise of how they get to the moments and how they are changed by the moments.
So, all in all, when characters develop minds of their own, the Control Freak in me is initially annoyed, but the Creative in me is utterly delighted. And when it comes to writing a novel, creative satisfaction is paramount.