After reading this entire thread, I couldn't help but think that everyone is way overthinking this one. Paladin has mega-Diplomacy. Group is captured, thrown in jail. Group starts making escape plans. Paladin calls over one of the guards and tell DM "I Diplomacize him!"
DM: Okay, like what are you saying?
Pal: Well, I tell him about the injustice. That if he doesn't help us get out, terribly evil things are going to happen.
DM: (Guard voice) I don't know. They did bring you in for breaking into the mayor's place and you all seem very dangerous. On the other hand, you're right about the weird things happening.
Pal: Listen, pal, we don't want to cause you any harm. But if we don't get out of here –
Wizard: and get our stuff back!
Pal: Right, get out of here and get our stuff back, then you and everyone in the town is doomed to die!
DM: Okay, make your diplomacy roll.
Pal: With modifiers, 40!
DM turns to wizard player: Your name is Brok. You've been working as a guard at the local jail for almost a year now. You are handfasted to young seamstress who is impressed by your good job. Your fiance had an ill mother that she was caring for, but the unspeakable demons killed her and nearly got your fiance, too! All you want is to get married, maintain a respectable job, and save a little money to pay off the debts on the family farm. A lot of weird things have been happening in town, and now the constables have brought in some odd looking guys that were caught causing a ruckus at the mayor's house. After talking with the blonde warrior guy, you get the feeling that they're probably pretty good people. You want to help them, but of course you have to think about your job and upcoming marriage. What do you do?
There we go. Helpful guard, but not too helpful. No DM fiat needed either way. Of course, you have to make it clear to the other players that poor roleplaying of the guard/king/wizard means that you as DM will handle it next time, and the person will be very risk-adverse.
Added benefit: involves the rest of the group more with the Diplomacizing.