Just one more vote for flexibility. Even if you have planned ahead how things will progress, remember that the players may very well come up with stuff you never came to think of and go in a whole other direction. When that happens your options really boil down to either rolling with it and changing your plans, or heavy-handedly herding them back to the railroad tracks.

Of these the latter may be easier to do, but it may seem to the players like their choices don’t matter and that if that’s the case, it’s no longer them playing a game but rather just listening to a pre-written story.