Editing Unwritten: System Reference Document/Running the Game (section)

===Deciding on Difficulties===

Like most of Unwritten, choosing the level of opposition is driven by narrative factors, rather than any attempt to represent a reality. Base difficulties off of both the nature of the situation and its importance to the story. It’s more an art than a science, but we have some guidelines to help you.

When deciding on passive opposition, look at how the PC’s skill level compares with the opposition. 
* 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.

Of course, there needs to be a good reason that backs up your choice in the story. It’s totally okay if that justification is something you’re about to make up, rather than something you know beforehand. Aspects are a great tool for this, especially situation aspects. It makes sense that a character with Just Not a People Person would have more trouble trying to communicate with natives who don’t speak their language. This is a situation ripe for compels, but if you aren’t specifically looking to throw a complication into the mix, adjusting the strength of the opposition makes sense.

In fact, the need for more challenge for a particularly skilled player or set of players is a great chance to be creative and bring new details into play. Let’s say you want to make bypassing an old D’ni security system more of a challenge, and you have an NPC you have been wanting to bring in. You could decide that the NPC has upgraded the system, or is somehow interfering. By doing so, you’ve brought in a narrative hook and made things more interesting.

You might also try using “out of place” difficulties to indicate the presence of unanswered questions during the game—for some odd reason, the store room you’re trying to break into has a +5 lock on the door. What could be so important in there that you don’t know about?

Either way, don’t skip the justification part—either let the players know what it is immediately when you tell them the difficulty, or shrug mysteriously and then let them find out soon thereafter (as in, the time it takes to think it up).