I've never understood the concept of characters running away in a story. Do you just not realise your preconceived plot and characters contradict until you're in the middle of writing it or something?
It very much depends on a lot of things. So far, I had almost complete control with Jack Diamond's first book, but they have gone a little off script on the second.
Strange Awakening stuck pretty close to the plan, except for a side-jaunt to New Orleans which was not planned initially. Oh, and the ghost that decided to get involved was not intended either, just appeared out of nowhere.
Between Worlds ... Liz wound up taking on a life of her own and becoming much more important than I'd ever meant her to be.
In one story I have not posted anywhere (tentatively
The Grey Files) , the MC is intentionally ... problematic (and both a lot of fun but also a lot of WORK to write) - he has no qualms about hijacking the story, misdirecting other characters, the reader or even the author. He's just that problematic. Especially for a guy who "just drives a cab" (Oh, and is possibly the last Mage left on Earth.... but mostly he just drives a cab - or so he keeps claiming, but he has contacts everywhere, knows more about the story than he lets on and ... well, he's fun but a lot of work).
Another story I've only finished two chapters on (Tentative:
A Subtle Distortion) has no real plan, so the characters very much are free-forming it. There's a rough idea where I want to go with it, but I let them - at least the three main characters presented so far - have their own head for a while before I reign it in.
Yet another story that I have pretty tightly plotted (tentative:
Digital Cowboy) still has a few deviations but nothing significant so far, aside from the introduction of a character who promptly dies but may very well return later... probably more than once.
And there's
From the Past, a story that went so far off the planned rails by chapter two that I had to restart it completely...