I agree with Scott. If you feel like your characters are dictating the story, then you're in a great place to write. Let them decide how to respond to situations you put them in, and if they make choices you didn't plot out, follow and see where they lead. There are so many permutations of character and plot that most first drafts don't have the right combination. I like to leave room in the outline for characters to surprise me. You can always go back and write the draft you intended, but you can't always get back to the place where the characters feel like they have a will of their own, i.e. Writer's Nirvana!

Finish. I know too many writers that never finish anything because of this very thing.

I actually just faced this same situation. My writing partner (also husband) and I are working on our latest action comedy script, and we've decided to NOT extensively plot this one ahead of time, to cut down on constraints and see what kind of humor will arise organically. My husband started the draft, and then I took over after he'd done five or six pages. I decided to restructure the beginning based upon something I thought was brilliant, and I picked up and wrote straight on from there. Well, when it was my husband's turn to write it again, he didn't agree with all of my restructuring. So he changed it back. In retrospect, he was right to leave it the way it had started out. I had some concerns about the story that had prompted me to redo the structure in the first place, but he was able to make those same changes and alleviate my concerns without changing the entire structure. We've passed the script back and forth a few times since then, and I've basically stopped rewriting anything at all. I know my most recent action scene isn't perfect, but going back and fixing it now might deflate that whole fragile "I love this scene!" feeling that I'm getting. So yeah, I think it's better to not rewrite until you get the whole draft out. Even if something doesn't seem like it's relevant (especially when you're writing a first draft in the manner that we've chosen), you may find when you get to the end that it is. Or not. Then you just edit it out. No big deal. :)

These guys have a great method: If your partner changes it twice, forget about it — http://www.amazon.com/Writing-Movies-Fun-Profit-Billion/dp/1439186758 De-rail, but still, finish that damn script!