Legacy of an Empire
When rolling the dice, you always try to beat a number — your level of success — to perform the intended action. what number you are rolling against can be determined in several ways, coming from either active or passive opposition or from another PC or NPC defending against you. Here are the different sources which can determine your level of success:
Passive opposition simply means that the player rolling the dice tries to roll higher than a fixed number. The number can represent some environmental obstacle, the difficulty of creating or doing something, or the difficulty of avoiding or overcoming another character’s skill rating who is not able or not aware enough of your attempt to pose active opposition.
In many cases the GM has to simply pick a plausible number to represent the difficulty of performing a certain action or overcoming an obstacle in the world. Players can influence this decision either by whining about the unfairness of the GM, or — more more productively — by invoking aspects to help their roll or make the challenge even more difficult.
In some cases — like when summoning Thermal Power or building a trap — it is up to the player to declare their own level of success. If they set it low, their chances of succeeding increases but the effect lessens, if high, their chances decrease but the effect become more rewarding.
Passive Skill Rating
Rolling against a character’s skill rating without them also rolling for active opposition, is sometimes called for in situations where the target of the roll is either unaware of the attempt — rolling against the passive Awareness when a guard does not expect anyone to sneak by and does not actively look for you — of if the target has no plausible way of actually doing anything against the attempted action.
Active opposition means another player or non-player character knows more or less what you are attempting to do and is willing and able to try and prevent you from doing it. The level of success of your roll is then determined by the result of their roll plus skill rating.
A Defend action only happens as a response to an Attack action or an attempt to Create an Advantage on the defending player. In both cases, a tie grants the aggressor a boost and a successful defend with style grants the defender a boost.
Other kinds of active opposition also require two players to roll against each other but don’t grant the player rolling active opposition any boosts.