FrogReaver
The most respectful and polite poster ever
I wasn't talking about established fiction. I'm talking about the prep that goes into an NPC. If I know he was an orphan and has a soft spot for that orphanage and its children, which is something the PCs can find out, then they are not going to be able to convince him to burn it down just by a roll. They would need something like hard proof that the kids were really imps or dopplegangers or something.
I think you may be putting the cart before the horse.
If an NPC is established to the DM's satisfaction whether in notes or the fiction that he would not burn down the orphanage then he would not do so. One of the major questions of this thread is whether the DM should use such predetermined behaviors for their NPC's or whether if on game night it might be more fun to have an NPC react a different way.
Now what your question does a good job of is raising the point that it is often desirable to have NPC's unable to be influenced to do certain things. That's a point I think most everyone here agrees with.
So I don't believe your scenario is gong to highlight any important point that hasn't already been highlighted. I really think the interesting part of this discussion is about when an NPC should be designed such that PC's have no chance of success / a 100% chance of failure when it comes to performing those triggering actions.
I think one factor in that regard that hasn't been mentioned is failure that results in nearly the status quo is alot different than failure that escalates the scenario to threatening the PC's lives. I would say that auto-failure that doesn't drastically escalate the stakes in a scene can be introduced at-will with no downside whatsoever. This covers NPC's like merchants giving away their wares and kings giving away their kingdoms.
But I think the other category is one we really should spend more time exploring.