Control of the Character
There are (at least) two issues here: character control and character change. Let's look at each one in turn.
First, let's draw a line between control and influence. When an NPC blackmails my character into murdering an innocent child--he's not controlling my character. I (the player) still have complete control over the character's actions. However, the in-game situation definitely influences which decision I might eventually make.
But, what happens when that influence becomes so overwhelming that it robs me of any real choice? Is this influence, or is it control? I'm not sure. Heavy-handed influence is probably not as interesting, story-wise, but it doesn't feel like the GM is violating my character's integrity either. At least, as long as it's a rare occurrence, and it flows naturally from the story.
Then again, lets turn this discussion around. As the GM, I often want the ability to manipulate the characters. The classic example is a fear check. When I throw a hideous beast from the pits at my characters, I want something in the game mechanics that helps portray the quaking-in-boots fear that the characters should feel. Many games contain "fear check" mechanics for this very reason. However, each time I force someone to make a fear check, I am forcing my (or at least the game system's) will upon their character. I am, at least in some small way, violating the character's integrity.
Same thing applies to mind control, or--to take an extreme view--combat. If the system imposes a -4 modifier on actions due to injuries, then it's simply influencing my choices. But, knock my character unconscious or (heaven forbid) kill him, and I suddenly lose all control.
And then there's the sticky issue of PC Immunity. In many games, a character's social skills can influence the behaviors of NPCs, but these same skills do not work on the player characters. No matter how high your manipulation attribute and your fast talking skill, you have to convince your friends based strictly on the strength of your argument. The game mechanics won't help you here.
This can easily become an unstoppable-force/unmoveable-object situation. My character is a master con man. His core image is based on his ability to manipulate others, yet I cannot effectively manipulate the people I interact with the most, the other PCs. Their characters' immunity harms my character concept. On the other hand, if you let me manipulate their characters, then we are stripping them of their power. You can't have both. Someone must lose.
Here's the real problem--we cannot avoid some amount of external influence over our characters. And this is not necessarily bad. Few people object to combat, mind control or fear effects. But when it comes to seduction rolls, that seems to cross an invisible line.
I don't think we can really quantify the difference between good manipulations and bad manipulations. Indeed, I suspect the line will change depending on the level of trust between the players, the type of story being told, and the personalities involved (both player and character).
My only recommendation is this: as much as possible, move the mechanics from controlling to influencing. Often you can accomplish this by adding temporary personality attributes, like "afraid of hideous monster (4)", or "desires Betty Lou (2)". Then reward the character for acting according to these personality traits, and give them negative modifiers whenever they struggle against the attributes. The player still has complete control, but the mechanics now exert some level of influence.
This method works really well for things like fear checks. The player gets a bonus to rolls for all-out defense, or for running away (or possibly for attacking if he gets backed into a corner). He gets negative modifiers for any direct action against the source of his fear. And this makes sense. I can run faster scared than you can angry. Meanwhile, trembling hands make it hard to aim.
But other things, like mind control and seduction, don't always fit this mold. Mind control seems more like a binary state. Either I'm reduced to a big meat puppet, or I throw off the psychic shackles and I'm free. Any struggle for control should be treated as a conflict and given the attention it deserves.
Similarly, seduction, fast talking, and other social skills--the personality system might work, but I have my doubts. Let's say Betty Lou successfully seduces me. If I walk away, when does the negative modifier apply? I guess I could get a negative modifier on any action due to pent up frustration and distraction (at least until I get a cold shower)? Hmm. That might work. But, if I agree to her advances, what does the positive modifier mean? Probably best not to think about that.
And what about fast-talking. Matchstick Mark makes a successful roll and convinces me to invest in his emu-farming scam. Now, how do we model this using positive and negative modifiers?
To be honest, social manipulation skills are really a pain in the butt. But they're a necessary evil. We don't expect the swordsman's player to actually know anything about sword-fighting. Yet we do expect a certain amount of wit and charm from the roguish swashbuckler. Social manipulation skills help us play characters that are more dashing and more witty than our real selves. Yet, no one is satisfied when a character (PC or NPC) is convinced by dice alone.
I'm not sure what the answer is, but I feel that the same basic rules should apply to both PCs and NPCs. That the same mechanics should be used to battles with both blades and wits. And that this probably means giving up some control of my characters--indeed, some games force players to give up a considerable amount of control, and some manage to pull it off with a considerable amount of style. But I still feel the rules should lean more towards influencing a players choices (or better yet, providing the player with choices), rather than controlling his character outright.
I've said it before, and I'll probably say it again. No character concept survives contact with the story.
No matter how much time and effort I put into my character, simply interacting with the game world will change him. At the very least, he will gain an emotional attachment to the NPCs and PCs he encounters. He may despise them and secretly plot their demise. Or he may love them and wish to protect them, even at the cost of his own life. Regardless, my character will grow and change. And this is a good thing.
But here's the interesting bit. This is really just another example of change driven by an outside influence. Why is this change good, but having a GM "violate my character concept" bad?
The main difference seems to be, in one case the game world (through the GM and other players) offers a stimulus to change, but I (as the player) stay in control. I decide when and how my character will react. However, when someone violates my character concept, they are forcing me to change against my will.
But I think it's more complicated than this. We, as players, seem to have an acceptable level of change. And, for the most part, this level is dynamic. As we become more emotionally involved with the story, we are more willing to accept changes to our original character concept.
So, part of the challenge might become very simple. If I, as the GM, want the players to respond, I need to engage them emotionally. Of course, that's easy to say. How do I actually do that? Let's look into our bag of GM tricks and pull out the trusty plumber's helper--let's introduce conflict.
So, you want the characters to be afraid of an NPC. Don't just describe the ichor dripping off the thousand worm-like tentacles writhing from its unholy maw. Descriptions, while they can build and enhance the mood, do not create a real emotional reaction. Instead, grab them with some sort of conflict. Have the beast threaten something the characters care about. Perhaps he has a hostage. Perhaps he has the power to inflict suffering on the innocent. Perhaps he possessed your character's mother, and is using her to confront the party.
The players may not react the way you wish. That is their prerogative. But they will react. Combine this with the temporary personality modifiers we discussed earlier, and you're offering a highly-charged opportunity for change. The players are more likely to positively engage with this opportunity, and incorporate the idea of fear more fully into the story. And that's what we really want. We want the characters to buy into the story and positively contribute to it.