Editing Unwritten: System Reference Document/Character Creation (section)

===Adding Stunts in Play===
You get three stunts for free, which you can fill in at any time. You can fill in other stunts at any time, but you must pay a fate point for each one. That’s because your refresh tells you how many fate points you start the game with, so by taking a stunt, you should have started with fewer.

If you’re out of fate points, but want to note down a stunt you have because you’re suddenly struck with the idea, do so. But your character doesn’t actually have it until you gain a fate point and spend it.

You’ll also need to reduce your refresh by one for the next session for each extra stunt you take.