Into every game, a little mind-controlling the PCs seems to fall. And why not? It ensures they’re fighting one of the scariest things on the field, it forces them to change their tactics, and it gets one or more of them out of your hair. But when we’re doing this, we still need to remember Rule 0 and make sure everyone, even whoever’s playing the mind-controlled PC, is having fun. These rules may not guarantee it, but they should definitely help ensure it.
Rule #1: Make being mind-controlled interesting. There are two ways to do this—either by allowing for some room to try to subvert it, or by saying “The spell says you’re going to do this to the best of your ability, but I’m going to trust you to make the decisions.” As a rule of thumb, if the player isn’t getting to make any choices at all or have any effect on the process, and this is likely to last more than fifteen minutes realtime for reasons other than that the mind-controlled players dice despise him, you’re doing it wrong. And remember that, especially after everyone knows the character’s mind-controlled, some sort of room for small rebellions rarely hurts, and often adds to the drama.
Rule #2: If you can get some level of character cooperation, do so. I recognize that in the rush of battle, this isn’t always feasible. But sometimes it is—and sometimes outside of battle, when there’s potential for the joys of social dealings with a mind-controlled PC, you might get players who will jump at the chance. Particularly if you’re keeping Rule #1 in mind, this is likely to at least keep the PC in question satisfied. Which brings us to….
Rule #3: Especially if you can’t follow the first two rules, leave some way to try to do something about it. Most systems are actually pretty good at this, since game designers understand that the illusion of control is an important part of the RPG experience (particularly for those people who play for escapism), and having even that control taken away is Not Fun. Either they’ll have a way for the PC to fight it, or they’ll have a way for the other PCs to try to do something. For those systems that don’t, you may want to houserule in some wiggle-room. If you explicitly prevent the PCs from doing something in contradiction to established rules, don’t be surprised if your players attempt to revoke your GM license.
Rule #4: If it’s preventing fun, limit the duration. Time-jump. Fast-forward. Drop someone a loophole (because it’s always cooler when it ends because of something the PCs did rather than Just Because). Have whatever’s causing the problem decide it’s had enough fun. If this goes on more than half a session, and people aren’t somehow enjoying it, it’s too long.
Mind control can just as easily make an awesome session as an awful one; make sure you know how to use it!