When a character uses
Deception,
Diplomacy,
Intimidation, or
Performance to influence or impress someone whose level or Will DC you don't know, estimate the level of the creature and use that DC. A regular citizen is usually level 0 or 1. Don't worry about being exact. It often makes sense to adjust the DC based on the target's attitude for Deception, Diplomacy, or Performance, making the DC easy for a friendly creature, very easy for a helpful one, hard for an unfriendly one, or very hard for a hostile one. You might adjust the DC further or differently based on the PC's goal; for instance, the DC to Request something an indifferent
NPC is fundamentally opposed to might be incredibly hard or impossible, and it might be easy to convince an unfriendly creature to do something it already wants to do.