Sometimes a non-player character has a set reaction to the PCs. Perhaps she’s automatically their enemy, or perhaps the party has rescued her, and earned her gratitude.
But there will be many NPCs that don’t have a set reaction. When the PCs request information or aid, it might go smoothly or it might not go well at all. Negotiation with a stranger is always an unknown quantity to the players it may be so for the GM, too.
When in doubt, the GM should secretly make a Situational roll. If the PC in question has a trait that can affect a stranger’s reaction, this should grant a +/-1 (or more) to the result. Examples include Appearance (which could be an attribute, gift or fault), Charisma, Reputation, Status, and such habits as nose-picking or vulgar language. The Reaction roll can also be modified up or down by circumstances: bribes, suspicious or friendly nature of the NPC, proximity of the NPC’s boss, observed PC behavior, etc.
The higher the Reaction roll result, the better the reaction.
On a Fair result, for example, the NPC will be mildly helpful, but only if it’s not too much effort. She won’t be helpful at all on Mediocre or worse results, but will react well on a Good result or better.
Example: Nathaniel needs some information about the local duke, who he suspects is corrupt.
He has observed that folks are reticent to talk about the duke to strangers.
Nathaniel decides to approach a talkative vegetable seller at the open market. Nathaniel has an average appearance (no modifier), but is charismatic: +1 to any Reaction roll. He makes small talk for a while, then slowly brings the duke into the conversation. The GM decides this was done skillfully enough to warrant another +1 on the reaction roll.
However, the situation is prickly: -2 in general to elicit any information about the sinister local ruler. This cancels Nathaniel’s bonuses. The GM rolls in secret, and gets a Fair result. The old lady slips out a bit of useful information before realizing what she’s just said. At that point she clams up, but Nathaniel casually changes the subject to the weather, dispelling her suspicions. He wanders off to try his luck elsewhere.