Sometimes I just want to say "He ain't buying it" when a player tries to con an important NPC or when somebody appeals to common sense in an affaire d'honneur. But I like to play fair with my players. I roll in the open. I like to avoid GM fiat when I can. Since Persuasion is going to be as ubiquitous in the Regency/Gothic setting as Shooting is in Deadlands, I think some guidelines are in order.
On page 62 of Savage Worlds Deluxe, the "Modifiers" subheading under "Trait Tests" advises a +2 bonus to easy Trait tests, a -2 penalty to difficult tasks, and a -4 penalty to very difficult tasks. Examples in the "Situational Combat Rules" section later on (like the "Suppressive Fire" example on page 71) show that a combination of penalties can exacerbate the penalties to -6 or more. Let's extrapolate from that.
- Suggested course of action is in NPC's best interest - +2
- Suggested course of action is not in the NPC's best interest: -2
- Suggested course of action is actively against NPC's best interest: -4
- Suggested course of action violates social mores: -2