So, is it possible to lose control over your story? What happens when a character does something you weren't expecting, or turns rogue and hijacks your plot completely? Do you reign them in, sit them down, and give them a thorough tongue-lashing? Or do you let your curiosity get the best of you and give them free reign to see where they'll lead?
In the novel I began during last year's NaNoWriMo, I thought I knew my plot and characters pretty well. I knew who was going to be killed, and who was going to do the deed, and when it all would happen. The intended victim, Grace, was home alone during a snowstorm while her husband was in town at a meeting. She'd stayed home because she hadn't been feeling well. The roads were bad, he was late, and she was becoming anxious. She looked out the window, then went to the kitchen to make a cup of tea. Suddenly, just as the tea kettle begins to shriek, she hears a noise, feels someone grab her, and falls to the floor, dead.
But hold on...that's not the killer bending over her, it's her husband. He'd arrived home, heard the tea kettle, and walked in the door just in time to catch Grace before she hit the floor. So, what just happened? Where's the killer? Did he run his car off an icy road so he couldn't get there before her husband did? Did he get the wrong house? Or did my characters just hijack my story? Maybe Grace decided she didn't want to get killed off. As it turned out, I liked Grace and was glad she hadn't died right then, so I developed a subplot for her and her husband and a different character emerged as the killer...someone nobody would have suspected. For that matter, I hadn't suspected them, either. The story took a completely different path from the one I had originally set out on. The basic plot didn't change, nor did the ending, but the route my characters traveled to get there did.
Sometimes, giving your characters the freedom to develop on their own can work for you. Other times, not. It's fine if you know what the characters are doing, where they're going, and if it all fits within the parameters of the story you want to tell. It's not so fine if they lead you into a muddle of events and subplots that turn your story into a chaotic, rambling, jumbled up mess with no way out short of resorting to deus ex machina, defined by the American Heritage Dictionary of the English Language as "An unexpected, artificial, or improbable character, device, or event introduced suddenly in a work of fiction or drama to resolve a situation or untangle a plot." Nothing screams amateur like resorting to this device to make everything turn out right. If your characters and plot are starting to look more like a tangled mess than the beautiful tapestry you were weaving, do what knitters and weavers do: rip it out--at least back to where it veered off track--and start over.
Writing requires creativity, but it also requires discipline. When all is said and done, you're in charge. You can allow your characters to have input into the story, but don't let them get carried away. As the author, you are in charge and all final decisions about what happens, and to whom, are yours.