With NPCs, there are no wrong answers. Can Charm Person be used to turn the bandit into a loyal henchman, or will the spell be broken as soon as you stab his friend? We can argue about one method or another being more fun, but in the end it doesn’t matter which method you use. The NPC isn’t going to feel like they’ve been treated unfairly. But a player might.
Lately, my most consistent D&D game has been Saturday morning Necrocarserous with John Bell. My character, Urlar of Yellow Waters, has been possessed by dragon spirits, dominated by an intelligent item, and affected by a spell which convinced me that my companions had been corrupted and that the only way to give them peace was to kill and eat them. Far from feeling like I was being treated unfairly, these moments where I lost normal control over my character are some of the most entertaining highlights of my time as a cog in the Necrocarserous Program.
My character didn’t turn into a temporary NPC while I sat on the sidelines and watched the game unfold. Nor did John ever tell me what specific actions I had to take. Instead, my character was given a goal. With the Dragon spirits, it was fairly broad. “Act in the best interests of the dragon cult” or something. Being empowered by the dragon spirits, I actually got a ton of boosts to my abilities, and a whole cult worth of people doing my bidding. Unfortunately, the rest of the party was in the process of robbing the dragon cult. Obviously I’m on the side of the party, but right now, Urlar is on the side of the dragons. So I ordered ‘my’ cultists to destroy the stone dome the magic user summoned to protect himself. The MU was the one actually in possession of the cult’s property, and thus should be our primary target. When a hole opened that was big enough to shoot arrows through, I insisted the cultists continue to focus on destroying the dome. A small hole, I reasoned, only allows a single arrow to fire through it, but a large hole can allow a steady stream of cultists into the dome to overwhelm the intruder. Eventually, my intentionally bad tactics allowed the party to withdraw successfully. (I don’t recall how they extricated me, but they did. Once I got back to town I had the tumor that allowed me to see dragon spirits removed.)
Being mind controlled in Necrocarserous isn’t something that happens to you. The player is not a passive participant. Mind control is a puzzle that can be solved. The player knows what their goal is, and they are obligated to pursue it. (John often tells me ‘no,’ and clarifies the nature of my goal when I try to deviate too much.) But any path to the goal that makes sense will be accepted. And thus can a mind controlled player minimize the damage they do to the party’s goals. Or even work towards the party’s goals if they are particularly clever about their reasoning.
This is how I want to run mind affecting spells from now on. As a complication which forces the player to stretch their ingenuity to the limits, rather than a buzzkill that bums the player out.