On the Copyblogger blog, Brian Clark lists ten mental blocks to creative thinking. I believe most writers are pretty good at overcoming these blocks, but one that stood out was "avoiding ambiguity." In the context of writing a script or book, we do tend to want to figure out the story as soon as possible, so we can get to the fun part. Often this means we stop exploring too soon and miss out on a story that could be even better.
Here's a tool you can use to explore your story: turn a piece of paper sideways. At the very left, write the starting point of your story (for example, "Brain gets a phone call telling him that his brother has been in a car crash"--this doesn't need to be what's on page one, but it should be the incident that kicks off your main story). By the right-hand edge of the paper, write how the book ends--if you know it. If not, leave that blank.
Now create three tracks of action leading from the starting incident and heading right. One progresses along the top of the page, one along the middle, and one along the bottom. Explore a different set of responses and actions in each one. For instance, let's say I know this book is going to be about how two brothers who are estranged from each other end up forging a relationship. In track one, Brian ignores his brother's plight at first; in track two, he tries to take advantage of it by taking over his brother's role in the family business; in track three he moves in on his brother's girlfriend, taking advantage of her vulnerability. Just keep playing with alternatives and see where they lead you.
You may find ultimately the story you want to proceed with will be comprised of bits and pieces of all three tracks, and incorporate elements you wouldn't have thought of otherwise.