In addition to your character aspects, game aspects, and the situation aspects that the GM presents, you have the ability to create, discover, or gain access to other aspects as you play.
For the most part, you’ll use the create an advantage action to make new aspects. When you describe the action that gives you an advantage, the context should tell you if it requires a new aspect or if it derives from an existing one. If you’re bringing a new circumstance into play—like throwing sand in someone’s eyes—you’re indicating that you need a new situation aspect.
With some skills, it’s going to make more sense to stick an advantage to an aspect that’s already on some other character’s sheet. In this case, the PC or NPC you’re targeting would provide active opposition to keep you from being able to use that aspect.
If you’re not looking for a free invocation, and you just think it’d make sense if there were a particular situation aspect in play, you don’t need to roll the dice or anything to make new aspects—just suggest them, and if the group thinks they’re interesting, write them down.
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So, if you don’t have any aspects made up for a scene or an NPC, just ask the players what kinds of aspects they’re looking for when they roll to create an advantage. If they tie or succeed, just write down something similar to what they were looking for and say they were right. If they fail, write it down anyway, or write another aspect down that’s not advantageous to them, so as to contrast with their expectations.
Secret or Hidden Aspects
Some skills also let you use the create an advantage action to reveal aspects that are hidden, either on NPCs or environments—in this case, the GM simply tells you what the aspect is if you get a tie or better on the roll. You can use this to “fish” for aspects if you’re not precisely sure what to look for—doing well on the roll is sufficient justification for being able to find something advantage-worthy.
Generally speaking, we assume that most of the aspects in play are public knowledge for the players. The PCs’ character sheets are sitting on the table, and probably the main and supporting NPCs are as well. That doesn’t always mean the characters know about those aspects, but that’s one of the reasons why the create an advantage action exists—to help you justify how a character learns about other characters.
Also, remember that aspects can help deepen the story only if you get to use them—aspects that are never discovered might as well never have existed in the first place. So most of the time, the players should always know what aspects are available for their use, and if there’s a question as to whether or not the character knows, use the dice to help you decide.
Finally, GMs, we know that sometimes you’re going to want to keep an NPC’s aspects secret, or not reveal certain situation aspects right away, because you’re trying to build tension in the story. If the PCs are investigating a series of murders, you don’t exactly want the culprit to have Sociopathic Serial Murderer sitting on an index card for the PCs to see at the beginning of the adventure.
In those cases, we recommend you don’t make an aspect directly out of whatever fact you’re trying to keep secret. Instead, make the aspect a detail that makes sense in context after the secret is revealed.