It happens to me all the time. I start to write a story, and before I know it has spiraled off into something I feel I have no control over. Take my current endeavor for example. I started off writing a story about a conflict between sisters, and the current revision is a political drama. Politics? I don’t know nothing ‘bout writing on politics! What’s a writer to do?
The best way I have found so far is two fold. The first step is to seek outside assistance. If you like the turns your story is taking, but not sure how to continue, simply ask a friend for help. Sometimes the solution isn’t really as dire as you make it out to be, and a fresh pair of eyes will figure that out. Or look to the blogs out there (and here) that are filled with writers giving advice. Or read books in your genre that deal with these kinds of problems. Or join a writer’s group and have a brainstorming session. There are many ways to deal with this part of the solution.
The second step is to add in things you know. Again, I look to myself as an example. I know very little about politics, but I do know a fair bit about the Civil War. At its heart it was about the politics of the day, and had a fair bit to do with the ‘division of a house,’ to paraphrase Abe Lincoln. When I started to feel overwhelmed with the political nature of a certain scene, I look to my trusty and well-used stash of Civil War books for reference.
Now, you won’t see any direct references, like a character named Stonewall or a bearded president with a desperate need for a sandwich (especially since their leaders are women). But, you might find an indirect correlation on how the two factions, split between two enigmatic leaders, differ on certain subjects they are more than willing to fight over. This helped me a great deal in getting the political aspect of my story off the ground. One problem down, only a million to go.