Editing Unwritten: System Reference Document/Aspects and Fate Points (section)

===Deciding When to Use Mechanics===

Because aspects tell us what’s important, they also tell us when it’s most appropriate to use the mechanics to deal with a situation, rather than letting people decide what happens just by describing what they do.

GMs, this comes up for you most often when you’re trying to figure out whether to require a player to roll dice. If a player says, “I climb this ladder and grab the idol,” and there’s nothing special about the ladder or the idol, then there’s no real reason to require an action to grab it. But if the situation aspects tell you that the ladder is a Rotting Rope Ladder and the idol is Protected by Deadly Traps, then you suddenly have an element of pressure and risk that makes it worth going to the dice.

Players, this comes up for you most often when invoking your aspects and considering compels. Your aspects highlight what makes your character an individual, and you want to play that up, right? So when the opportunity comes up to make your character more awesome by invoking, go for it! When you see an opportunity to influence the story by suggesting a compel for your character, do it! The game as a whole will be much richer for it.