yep, the most you can do is have an outline of how you want the story to go. When you actually start writing it, things are subject to change and you also have to fill in any gaps and make connections that wasn't in the initial conception. Just like when you're playing a game, you may have any idea of what you have to or plan to do, but once you're playing, you can usually follow in that direction, but things will not play out just the way you imagined, or you find that you missed or forgot a detail and have to "wing it".