Rolling back to this again, because I think this, for me is the biggest point.
Let's alter the situation a bit. Instead of a charismatic character, Doppleganger Ahnehnois is playing a combat character. Half orc barbarian with a great axe.
He meets the raging warrior and decides to engage by planting his axe in the forehead of the raging warrior. The DM responds in an identical manner: "The raging warrior is not interested in fighting. You cannot fight this raging warrior. Nothing you do can engage this guy in a fight."
Is that an acceptable DM ruling? What's the difference between that and the original DM?
Yes.
It's an odd one; hard to imagine a rationale for it. But it's within the letter and intent of how the game works. I could imagine that the NPC is some kind of higher power who can't be attacked, or that the DM simply is trying to censor problematic behavior, or maybe some other reason.
The difference is that it's a stranger example. It's also not really equivalent; a negotiation requires two active participants, while a battle only requires one. It's much easier to imagine a negotiation being unfeasible because it is a more sophisticated interaction.
But let's turn this example around the other way. Let's say that I had wanted to engage the NPC in dialogue, had said my full spiel, and rolled a Diplomacy check. Let's suppose that that check was really high, high enough to mandate an attitude change under the rules. And then let's say the DM ignores this, says that the NPC refuses to listen and attacks before my character gets a word out, ignoring his apparent desire to talk, and calls for initiative and starts rolling attacks. Can I, as a player,
force the DM to stop? Do I have a right to dictate the behavior of a character that the DM created and is playing, determine the amount of time that's passed, or decide how the skill mechanics are interpreted in this situation? If the DM goes on, am I supposed to file a protest? Walk out?
I think it's pretty obvious that the appropriate behavior in this situation is to accept that the DM knows more than I do, and accept that I asked him to DM for a week to give me a break, and let him do his job. If I stopped the game every time I disagreed with a DM's decision, it wouldn't end up being much of a game.
See, to me, the DM has declared unilaterally that a particular encounter must be resolved in a specific way and player choice has been removed from the equation. The DM has decided that this encounter will be a fight, no matter what.
Yes, that's what the DM decided. A much more reasonable decision than in your rather extreme counterexample, if still not one that I was wild about. From what I knew about the NPC, he had insufficient motivation to be so set on that outcome. But ultimately, I don't know and can't control what the NPC was thinking.
And frankly, this ruling is minor compared with all the other things the DM established to create the encounter in the first place. All of which were equally his prerogative.
I didn't have any say in whether the opponent existed in the first place, or whether he had hostile intent. I didn't even have any say in what my character looked like, oddly enough (I was mind switched to a new body; no dice rolls involved). I didn't control where this encounter took place, or when, or how many people would be there, or what time of day it was, or what the weather was, or what the broader geopolitical situation was that this NPC was living in. Nor did I control any of the factors that might influence this NPC's behavior other than my attempt at talking to him with my character. And, as it turned out, I had virtually no influence on the outcome of the encounter; I simply rolled an initiative and was critted to the point of near death without making another die roll; less luck with stabilization rolls and I likely would have been dead, which if I know this DM might very well have stuck.
All of which is perfectly fine.
The purpose of this example is to show that I'm not hypocritical or domineering about these things. It's not like I don't understand the player's perspective or am hostile towards it. I simply acknowledge that the game works best with one central authority. That's a lesson that transcends my personal game, which is why it's in the rulebooks.
Of course, a wizard casting a simple Charm Person spell ends the fight immediately (assuming a failed save) and can do what the non-caster cannot do - reframe the encounter into a non-combat encounter. Unless, of course, there are those here who would rule that Charm Person cannot stop someone from attacking. That being friendly or helpful won't actually change anything in the encounter. After all, that's precisely what this DM did do for the charismatic character. So, it should be ruled the same for spells, shouldn't it?
Not necessarily. After all, a spell and a conversation are not the same thing. Charm Person is actually mind control, in a minor but still significant sense. I see no reasonable expectation that a particular scenario should play out the same way with a Charm as with a Diplo.
Could a DM say that a particular NPC was immune to Charm, or apply a bonus to his save so that he makes it? Sure. Could a DM decide that even a successfully NPC still engages in confrontational behavior? Sure. It's possible that an NPC might consider it acceptable to attack or imprison a friend under certain circumstances. Can the DM decide that the character simply can't cast the spell, or that it does nothing? Absolutely, for any number of reasons. Most of the time, a successful Charm probably accomplishes the goals of the caster, but there are no absolutes here.