1) Know how the minds in your story work. For example, if the people in your world use a division of the mind into id, ego, and superego, someone trying to possess your main character might be able to control their ego and superego, thus controlling the public face they present to the world, but the id would remain free and a potential source of trouble. Or vice versa, of course. That would be a slow, interesting way to make the character descend into insanity, actually: work from below and very slowly erode their control over the most selfish parts of themselves, while searches of their conscious minds would reveal nothing and the shift in personality would seem natural.
Perhaps you disdain anything this simplistic. That’s perfectly fine. What you need to know is how much of the mind is really available to the controller or the possessor, or subdued by hypnosis.
Why? Because it affects what the other people in your story can be expected to notice and what the controlled character can do about it. If a mind-controlled character regularly goes into fugue states and loses all memory of their actions while under, then, hey, that might be a great shrieking tocsin that something is wrong here—especially in a world where mind control is known to exist. If a bit of the character is left trapped and helplessly watching the actions of her body, then she might stand a chance of escaping that she won’t if she’s completely subsumed. And then there are the more extreme forms of what can happen to a mind. No one is coming back from what the slake moths do to them in Perdido Street Station, because their minds have been liquefied
Once you know how minds in your world work, you can answer other basic questions like: What happens when mind control is disrupted? How strong is the basic grip of the person doing this? If this is normal hypnosis, what happens to the people who are not readily suggestible? And so on.
2) Remember not to make any means of control infallible. Otherwise, these people would already have taken over the world, and even if you’re writing a secret-conspiracy-they-discover form of the tale, any battle the protagonists conducted would be foredoomed.
Perhaps mind control can be fought by those who have empathic and/or telepathic powers. (Perhaps mind control is an extreme form of telepathy). Perhaps it leaves behind traces that are easily recognized by a knowledgeable person. That’s one of the things wizards in Jim Butcher’s urban fantasy series do: their Sight sees the true psychic state of a person, which includes recognizing psychic wounds inflicted by mind control. The wizards promptly hunt down and execute the responsible parties.
Perhaps the hypnosis has limitations. If you’re planning on using a hypnotist with no magical powers, I would certainly suggest that. (Please pardon the bad pun). Fantastic claims aside, hypnosis cannot make people do things to which they are fundamentally opposed, such as making them murder someone when they deeply believe murder to be wrong. Nor can it readily affect people who are not very trusting; one of the tests used to determine receptiveness to hypnotic suggestion is to have the subject fall backwards into another person’s arms. Someone who can do it makes a much better hypnosis patient than someone who can’t. And people who can go deeply into somnambulistic states, so that the effects of hypnosis are permanent and strong, exist but aren’t the majority either.
What about possession? Well, as usually portrayed it seems to have no limitations; it can make its victims murder their loved ones, go on demonic sprees of terror, and fall victim to crosses. If you can’t bear to limit the strength of the possessor, limit the number of people who can do it and think about making it vulnerable in other ways. Distance is always my favorite for this, as for other mental abilities, because it’s so simple and yet can be used in a variety of ways. If the possessor can only control others from a mile away, people who knock him unconscious, dump him in the middle of a desert miles from civilization with no water or clothing, and then ride madly away are stronger than he is. He’ll likely die before he makes it into the vicinity of anyone he can control. Or they could be even smarter than that, and just kill him outright when they knock him unconscious.
Decide on the strength of these mental abilities early, and keep it consistent. Not doing that usually results in stories where the enemies are strong in the beginning, to build suspense, but then their abilities inexplicably weaken to let the hero/ine escape in the end.
3) Decide their place in the “ecology” of magic. For one thing: Who’s going to teach someone else to control minds? Even if an individual person with this ability isn’t bad-natured, I’d think it would make the people around her freaky. Can she develop and control it on her own? Does she do it accidentally, in the course of attempting to use her magic for something else? (The idea in Butcher’s books is that a lot of young wizards don’t mean to break their victims’ minds, but once they’ve done it, they begin to hunger after power to control other people and start down the road into black magic).
What’s the relationship of possession to mind control? Is it the same thing? What’s the mechanism for the control? Magic, telepathy, a process like hypnosis, an intense study of personality that attunes the person with the ability to just one other mind? Can people keep someone else from controlling them? Do they recognize the intrusion into their heads? What effect does the control have on the person exercising the manipulation? It would be fun to see someone who wasn’t just a Machiavellian mastermind, say a religious hermit who wanted his privacy to meditate on God and used his gift to drive fear into the hearts of those damn kids who kept trying to sneak up into the hills in search of adventure.
What good are these abilities? To commit crimes or take vengeance is the classic answer, but if they leave signs behind, then sooner or later someone will find out, and that’s not so good for the person controlling minds. Perhaps they should have tried good old-fashioned clever plans instead.
-Being the power behind the throne, changing minds in tiny ways to influence political decisions.
-Counseling, a way to introduce prohibitions against behaviors that people want to quit (some real-life hypnotists function this way).
-“Uploading” information; if a manipulator commanded someone else to have perfect memory, he might be able to learn a new skill faster or pass a complicated test.
-Putting on shows (another thing some real-life hypnotists do).
-Stopping criminals, and other people about to do something undesirable; imagine a manipulator on the city guard who could possess the fleeing criminal so that she turned her horse back around, or hold a person about to commit suicide hostage until someone else could physically reach him.
-Beginning the process of healing by letting people back away from their own emotions for a time, or embrace those they’ve long denied. (Please don’t mistake this for an endorsement of instamagical healing, which I hate. Limitations, remember?)
-War, especially messing up the command structure by making generals and key officers give the wrong orders.
-Spies. Assuming they can read any mind they take hold of, and do it fairly subtly, a team of manipulators could be terrifyingly efficient at figuring out where rebellions are fomenting—the Thought Police of 1984 embodied. They might also be able to make the potential rebel leaders march themselves to jail.
All of this still treads an uncomfortable ethical edge—as it should. I think these are abilities hard to write as completely neutral, unlike, say, creating illusions. People who work with the person they know can control minds might still be jumpy, no matter how many assurances of her good will they receive. But it’s more complicated than turning every character like this into a raving maniac or a greedy robber.
4) Know how you intend to depict mental battles. One of the major complaints in stories where a lot of characters are psionic is that mental battles are too fuzzy. So it’s a “titanic struggle”—but because these characters aren’t fighting with their bodies, the author is deprived of a great deal of her usual descriptive language, and she retreats into adjectives that could refer to anything. Or she tries to write it in the language of dreams. I think this is a mistake, unless the character fighting the mind control actually is asleep. The operations of the waking mind are hard to describe, yes, but many of them are a far different experience to the people inside those minds than dreaming is.
So: how to write it?
My favorite way is using analogues of physical conditions, based on fairy tales about two wizards who can both shapeshift, so that one takes the form of a grain of corn and one takes the form of a hen, and so on. The person trying to possess another might envision his attack as a tsunami, irresistible, but perhaps the defender dives to the bottom of the ocean of her own mind and hides. In this kind of battle, the person with the cleverest imagination wins.
Another way is to think about the physical structure of the brain. Can you describe the battle as flickering among synapses, lobes, ganglia? (This assumes a human brain, of course). Perhaps your characters don’t understand them in a scientific way, but they could still describe what they see.
You could also use their cultural conception of the mind. (This is where Point 1 is helpful, since hopefully you’ve already decided how minds, and mind control, work in your world). Many authors write about an astral Otherworld that has fairly consistent landmarks; why couldn’t a mental world be the same way? Everyone’s may be unique, but perhaps they share enough common features that the manipulator who intends to possess someone and make her into a helpless pawn knows he has to chop down all the trees in the Moral Forest.
If mind control is a big factor in your story, you should know how you plan to describe the time spent in people’s heads, battles or not. Description limited to “He was in her head” and “Then he bounced off her shields” gets old real quick.
And I suppose that, for now, that’s all I really have to say, since each point covers a lot of ground.