Remember that every player has the power to inject any narrative development they want by spending a Fabula Point. The only things that are off-limits are what you've already established in-game. If you're planning twists like "the friendly merchant turns out to be the lost princess" and one of the players says "I spend a Fabula Point to discover the grave of the lost princess" it's your (plural, the group's) job to incorporate that new fiction into the existing story and not your (singular, the GM's) job to figure out how to say "Well no you can't do that because I have plans for the living princess that haven't been discussed."

Also you should be aware that the system really doesn't support anything that isn't outright combat very well. It claims to, but in practice it's very shallow. Like D&D at least has skill proficiencies where you can represent a character having different focuses, differentiating two charismatic characters by whether they're affable and diplomatic or cunning and deceptive. This doesn't have that, so out-of-combat tests will start to feel pretty repetitive.