When you roll the dice, either you’re going to fail, tie, succeed, or succeed with style.
Every roll you make in a Fate game results in one of four outcomes, generally speaking. The specifics may change a little depending on what kind of action you’re taking, but all the game actions fit this general pattern.
If you roll lower than your opposition, you fail.
This means one of several things: you don’t get what you want, you get what you want at a serious cost, or you suffer some negative mechanical consequence. Sometimes, it means more than one of those. It’s the GM’s job to determine an appropriate cost.
If you roll the same as your opposition, you tie.
This means you get what you want, but at a minor cost, or you get a lesser version of what you wanted.
If you roll higher than your opposition by 1 or 2 shifts, you succeed.
This means you get what you want at no cost.
Succeed with Style
If you roll higher than your opposition by 3 or more shifts, you succeed with style.
This means that you get what you want, but you also get an added benefit on top of that.
For the GM: Serious Cost Vs. Minor Cost
When you’re thinking about costs, think both about the story in play and the game mechanics to help you figure out what would be most appropriate.
A serious cost should make the current situation worse somehow, either by creating a new problem or exacerbating an existing one. Bring in another source of opposition in this scene or the next one (such as a new opposing NPC or an obstacle to overcome), or ask the player to take a consequence at their lowest free level, or give someone who opposes the PC an advantage with a free invocation.
A minor cost should add a story detail that’s problematic or bad for the PC, but doesn’t necessarily endanger progress. You could also ask the PC to take stress or give someone who opposes the PCs a boost.
It’s okay if the minor cost is mainly a narrative detail, showing how the PC just barely scratched by. See more advice about dealing with costs on in Running the Game.
For the GM: How Hard Should Skill Rolls Be?
For active opposition, you don’t really need to worry about how hard the roll is—just use the NPC’s skill level and roll the dice like the players do, letting the chips fall where they may. There are guidelines about NPC skill levels in Running the Game.
For passive opposition, you have to decide what rank on the ladder the player has to beat. It’s more an art than a science, but there are some guidelines to help you.
Anything that’s two or more steps higher than the PC’s skill level—Fair (+2) skill and Great (+4) opposition, for example— means that the player will probably fail or need to invoke aspects to succeed.
Anything that’s two or more steps lower than the PC’s skill level—Fair (+2) skill and Mediocre (+0) opposition, for example—means that the player will probably not need to invoke aspects and have a good chance of succeeding with style.
Between that range, there’s a roughly equal chance that they’ll tie or succeed, and a roughly equal chance that they will or won’t need to invoke aspects to do so.
Therefore, low difficulties are best when you want to give the PCs a chance to show off and be awesome, difficulties near their skill levels are best when you want to provide tension but not overwhelm them, and high difficulties are best when you want to emphasize how dire or unusual the circumstances are and make them pull out all the stops.
Finally, a couple of quick axioms:
Average is called Average for a reason—if nothing about the opposition sticks out, then the difficulty doesn’t need more than a +1.
If you can think of at least one reason why the opposition sticks out, but otherwise just can’t decide what the difficulty should be, pick Fair (+2). It’s in the middle of a PC’s range of skills, so it provides a decent challenge for every skill level except Great (+4), and you want to give PCs a chance to show off their peak skill anyway.