The concept that bothers me about this discussion is having game mechanics that only work one way. A game mechanic should either work all the time or not be in the game. Having something that works differently for the GM as it does for players seems somehow to be "cheating" to me. I don't like "PC-only rules" and "GM-only rules".
It really isn't. The DM does a lot of things differently than PCs. Monsters work differently than PCs. DMs set DCs at whatever they think is appropriate so they can set a DC 25 or 30 to something they think should be near impossible. They don't have to follow "rules" to do that. The DM and players ALWAYS use different rules.
There was a section in the 4e PHB(or the DMG, I always forget which one) talking about social skills and players. It explicitly said that social skills do NOT work on PCs and you should never use them on them. That interactions between NPCs and PCs should be done via roleplaying and the PCs get to decide how to act in any situation. It suggested that you might use something like Insight to give the PCs INFORMATION but never to tell them what they think or do. So, making a an Insight check might tell you that the NPC is being a little shady and is concealing something, though you aren't really sure what. The player is allowed to decide that their character is trusting and just writes it off as the NPC being nervous or being a secretive person. Just like if they fail an Insight check, you can tell them that they can't tell if the person is lying or not. Then the player can decide that their PC thinks there's just too much evidence that the person is lying and confront them about it. It also says that in general, PCs should never use skills against each other at all. They just roleplay out their interactions.
Although that section was in the 4e book and there isn't a similar section in the 5e book, there was a similar section with a little less detail in the 3e book. It's just a good general rule to apply in D&D. Players don't like being told what to do unless there is a magical compulsion to do something. Even then, they'll likely complain about being forced to do things they don't want to.
Several posters in this thread have stated that influencing the behaviour of a PC is bad, that it removes player agency. I agree with them, though I wonder what the difference is between an NPC charming a PC and an NPC intimidating a PC?
One is magic, mostly. It's that it's easier for a player to wrap their head around. They can picture themselves walking up to a big, nasty orc and saying "I'm not afraid of you!" and that option is being taken away from them. But if someone says "Try as you might to kill the enemy, your mind won't let you. It's like you are trapped in a waking dream, your body moves on its own. It's MAGIC!" then it's easy to understand. Magic is...well, magical. it doesn't follow the laws of physics. It doesn't exist in real life so players can easily say "I've never experienced magic before...so I don't know what it would feel like to be charmed. I'll follow what it says in the book."
But people have had people try to intimidate them or convince them before. People FEEL like they were 100% in control of their actions at the time. They DECIDED whether to be intimidated or convinced. Whether they were 100% in control of their actions or not or whether hormones, adrenaline and various physiological effects on their body forced them to act one way or another is probably a debate better had by scientists or philosophers. However, people FEEL they were in control the whole time. They expect that same control in a D&D game.
However, I think that if the GM must respect player agency then the players must respect GM agency. For example, if NPCs are not allowed to intimidate PCs then PCs should not be allowed to intimidate NPCs
The real difference here is that the GM shouldn't really have "agency" in the same way that players do. It's a different sort of agency. Your agency is through the story, through multiple characters, and through your ability to decide rulings and guidelines on the fly.
If they players try to intimidate your NPC, you have a lot of choices in what happens. You set the DC on how hard it is to intimidate an NPC. You can decide that an NPC is unflappable and it takes a DC 30 check to succeed. You can decide what exactly happens when the PCs succeed in an intimidate check. Does the NPC give up his boss or does a successful intimidate check just cause him to commit suicide when he realizes he's going to die whether he gives up his boss or not?
The dirty little secret here is that social skills don't do anything at all unless you let them. People can succeed in a DC 30 Persuasion check and you can decide that the NPC still isn't convinced. The difference between the PCs and the DM is that the DM is supposed to see the big picture. The DM sees all the pieces and knows what is likely to happen next, what the plot of the adventure is, what the bad guys are planning, and so on. You can decide whether or not it would be fun to have the NPC be convinced or whether it would be more fun to send the PCs on a side quest to get something the NPC wants first.
Players don't think of the big picture. They don't want to. They want to think only of what their PC is doing or thinking at any one moment. That is all they have control over during the game so that is all that is important to them. They don't think "It'll be fun for me to be intimidated" they think "Really? The only thing I can do during a game is control my own character and that has been taken away from me."
I want a mechanic that allows for coercion in both directions while keeping some amount of agency. Is such a game mechanic possible?
I'd say no. Since even if the PCs intimidate an NPC, a VERY small portion of the DM's agency has been given up(the ability to control 1 character out of the thousands or millions he controls). Probably even 0 agency since the DM decided whether the intimidate worked in the first place. If the PC is told what to do by the DM then ALL of the player's agency has been removed.
