I've heard more than a few authors say that once their characters are fully developed they often will take over and drive the story and plot. Others prefer the characters, no matter how well developed, stay within the original borders of the story.
The preference may be dictated by the degree the author outlines their stories or if he just starts from the beginning and lets things happen from there. And certainly well developed characters in a series would govern any new story lines an author creates to avoid having them go against type.
Do your characters come alive and rule or does the story? Will a character cause you to change a story or plot in the middle of a book because it's something the character (as you've developed it to that point) would not do?
For me I would say that as I get going, I've gotten a whole lot more structured. So the characters pretty much go where I tell them at this point. At least in my main series. But then I'm also not slavishly plotting out their every move before hand. I've got goal posts and way stations along the way pre-plotted out and a few key interactions and telling lines I want to put in there to inform the story in the direction I want to go.
That said even in my largest series, where I need to do the most preplotting for the next book I'm writing, I still find myself laughing as the character goes or does something in a direction I maybe didn't envision. But its mostly small stuff, but despite that it is still generally things I will take and build on and weave into later books.
Now when I'm first starting out with a new book or story. I know where I want things to go and I start out writing it. However starting off learning a character in a way that lets me write and not write a boring info dump that no one, not even myself, are interested in reading, sometimes things will go in a surprising direction. but after a while things settle down. My characters don't hijack my stories. But sometimes to get a fun story you have to abandon a few of the original ideas along the way and insert others.
Its more a case of finding something good, when the alternative is writing and rewriting something boring but completely in line with your original idea until it sings. Sometimes its easier to just take the gold and run with it.
In my personal opinion, the more experienced you get as a writer the more your characters will just do what you orginally planned. Its a case of being a better writer. thus when you get to a potentially sticky situation you can write your way through it. Where a more green and inexperienced version of myself would probably have become stuck and wrote something funny and humorous that changed things slightly so that I could keep pounding my way through the book. Going deep into a series, where you don't just have to keep your original idea pure but also tie everything in that happened over 4+ previous books, also restricts the kind of hijinks you can allow your characters to get into.
Now, fully conscious of people like Neuro, pointing out the special and unique butter fly effect. I will add that I think most people most of the time will find their characters becoming more and more obedient as they write. But that of course some people put their characters in a straightjacket and plot everything out to death and gone first, allowing for no hijinks from their characters. While other people are probably almost pathalogically incapable of writing a story that they already know the end of and what's going to happen along the way.
But once again, most people, most of the time, will have better and more full control over their characters as they go along.
But that's just my 0.02c.
The Deposed King