There are many ways to define control, especially in the world of publishing. There is creative control or the control of the rights to what you've written, but I'd like to address the control over the characters and the storyline that you've spent time putting together. We all write a little differently. There are those of us that begin with an idea and let it grow as we write, feeling confined by the structure of a written outline. (This is me and my writing style). There are those that can't write the first word until they've worked out every detail of how the story will go. Each of these extremes, and everything in between, have their own forms of control that each writer feels most comfortable with. Whether you're a planner or if you tend to go with the flow of your writing, maintaining your chosen storyline and character arcs can be challenging in the midst of spinning your tale.
It is very easy to let the story begin to go in its own direction. I can attest to this as the novels that I've written have begun to change and grow, organically, as they should; however, there are times that your storyline or character can take on a life of its own. I have eliminated an entire character when I realized that he was no longer serving the purpose I had originally thought he would when I began writing.
When that happens, it is important to be able to step back and ask yourself, "is this what I wanted for this character/storyline?" If your answer is Yes, then great, go with it and continue. If your answer is No, but I kind of like where this is headed, then I'd say you're still falling under the umbrella of organic change as the story/character presents itself. However, if your answer is, No, I'm steering off course, then maybe you have the beginnings of a follow up novel/story/character.
I went off course, now what?
The question then becomes, what do I do when I realize that this has happened? Most importantly, do not delete all of that hard work, this has been a tough lesson learned in my own writing. It is helpful to pinpoint where this character/storyline went from a part in your current story to taking on a life of its own. Once that has been accomplished, take just that section out of the story by copy/pasting it into its own file.
Create a folder of snippets of work like this one and add to it every time you encounter this issue. Before you know it, you will have many characters and/or storylines that didn't work with something you've already written, but could become an important part of something you've yet to write.
A Word to the Wise
A word of caution, it is very easy, once you've identified the snippet from your current story, to copy/paste it into this folder, continue on your current project, then come back to it only to forget all of the thoughts that you had when you sent it there. Our intention is never to send an idea somewhere to die. Rather, when you're setting this snippet aside, do so with notes as to where you see this going and what may possibly be done with it.
Flesh it out a little bit, just enough to where you will be able to pick up with it when you have time to do so and turn it into your next piece or where in the future, when working on a piece, what you've created may just fit in perfectly. You will simply need to go to the folder with these snippets, copy/paste the one that works, then edit it to fit in with the piece that you're now working on.
Control is something that can be double sided. As authors, we want to see our stories grow out of our original ideas, but we have to make sure that we stick with the plot structure we created, for the most part. Having somewhere to place these other ideas will provide a montage of thoughts and ideas that may or may not get used in future writing, but it validates the writing that we've already done as well as gives us a library of ideas when and if we come across the "writer's block".