I am in the process of writing my first novel. I have written nine chapters so far. My first five chapters came to me without much planning. For my next five chapters, I have been working to a plan for each chapter. Before writing the novel, I had a very specific theme in mind. But I'm now nine chapters in, and the theme is not developing in the way I expected it to. So my question to you is this: Should I let my characters tell their own story (and hope that they reach a satisfactory ending without being told what to do by me the author!)? OR Should I be going back to my original theme, and slowly start trying to work it into some of the chapters that I've already written, and develop a more structured approach for the rest of the novel?
Are you happy with the first nine chapters? Are you satisfied that the direction the story is going, while not as originally planned, is a good one? If yes, why change now? If no, then yes, try something else. Personally, I don't plan anything. I don't go off willy-nilly, either, but I don't have outlines, character sheets, a theme in mind - nada. The story happens as it happens, and my only 'structure' is to make sure what happens fits what's already happened.
There is no "should". Everybody does what works best for them. And planning doesn't always mean "forcing" either. It just means that you do the thinking and let it develop before you write instead of simultaneously. That way you can spot problems right away and therefore avoid some of the rewrites.
I agree with shadowwalker. If you like the current theme/plot, why change it to the original one? You can save that for another novel, short story, or just some short piece, where you can practice you writing. What works for me is that I have a summit or the ending of the story in my head, and I try to write my way towards this original ending/summit. I write and see where the story goes, and if on some point, a story takes a strange unexpected turn, why not proceed in that direction if it's good? Plus, it's also a lot of fun, since you don't know exactly what is going to happen. I also wrote two short stories without any plot and theme, I just liked the sentences with which I began the story. I think it's good if your story takes unexpected turns, that's one of the major points why I write - it's fun that way. “I have a broad sense of where the story is going—I know the end, I know the end of the principal characters, and I know the major turning points and events from the books, the climaxes for each book, but I don't necessarily know each twist and turn along the way.” - George R. R. Martin
I tend to make a loose plan when I write. I know how I want it to begin and end, and I know a few important scenes that I need to happen. But mostly, I let the story write itself. I find that my biggest writer's block comes when what I'm writing isn't working. So I tend to delete what I was "forcing" and start that part over. I say if you're happy with what you're writing, the theme will develop as you go.