Nothing earth-shattering today; just some thoughts on the ever-popular Charm Person spell. It happens that in my intermittent game with my brother's family, my wife likes to play a dryad who makes liberal use of her innate Charm Person spell. So far she's only got one spell per day, but she keeps a retinue of the enemies she's charmed as sort of de facto henchmen, though I have set a limit of one per level plus Charisma bonus.
Anyway...Charm Person. What does it do, what doesn't it do, and what are its limits? The Moldvay Basic Rules tell us that a charmed creature will treat the caster as its best friend, and will try to defend the caster from all threats, real or imagined. It will obey most orders, but will resist orders that are against its nature (alignment and habits), and will refuse to kill itself. (That goes against the nature of almost all creatures, I would think.) Mentzer adds that any attack by the caster upon the charmed creature, whether physical or magical, immediately negates the charm.
So, obviously, a charm is NOT mind control. Charmed creatures don't simply obey like automatons. They really are, essentially, temporary henchmen. A charmed creature is loyal to and trusts the caster, up to the limits of its own nature. The fact that it's stated to resist, rather than outright refuse, orders that contradict its nature suggest that it can be persuaded to do so, though. I think it's important to note that while the creature's loyalty to the caster is stronger than all others, it does not automatically negate any previous loyalties.
A charmed creature wants to please the caster, just as one friend wants to please another. It will trust the caster's word. If the caster tells it something plausible, it will believe without question, and if the caster tells it something outlandish, it will at least believe that she honestly believes it herself and is not lying. Sometimes it won't agree, but it will take anything she says in the best possible light. It will not believe anything ill of its new best friend, and may go to great lengths to rationalize any behavior of the caster's which suggests otherwise.
Any simple request which has no obvious negative repercussions for the creature's own life or its previous loyalties will be obeyed easily.
A request for it to do something to which it ordinarily would be very averse will require some persuasion. The caster will have to explain why it's so important that the creature do this thing, or promise action to allay its unease, or both.
A request to do something that impinges on a previous loyalty will also require some persuasion. Ideally, the creature would prefer not to displease either side. Remember that charmed creatures are brainwashed, not stupid, and will eagerly suggest possible alternatives and compromises to avoid the discomfort of acting against their nature as well as that of displeasing their new best friend. The charmed creature ultimately will turn on its former allies, if it comes to that, but only after it has exhausted all other possibilities, or the former allies have clearly broken faith with it. Before that point is reached, if the caster insists on forcing the charmed creature to do something totally at odds with its inclinations, the creature should immediately receive another saving throw to escape the charm. (Not in the rules, but it should be.)
While a creature may be charmed without sharing a language with the caster, giving it orders is difficult. Depending on its intelligence, it may be able to interpret gestures and facial expressions, but that's crude communication at best.
Remember, too, that a charm does not change the creature's personality. A charmed monster does not necessarily become a pleasant companion. A charmed ogre is still a gluttonous oaf with an aversion to bath water, and a charmed evil priest is still a sadistic bastard. Also, the caster of the charm is the creature's best friend, but the same does not hold for the caster's other party members. A charmed monster will only be as civil and friendly toward another party member as it would normally have been, unless the caster makes clear that such strife displeases her. Even so, she might still have to endure her charmed ogre's endless pleading and litany of justifications for why it should be allowed to eat that plump, juicy halfling. He'll even offer to share with her.
An example from an actual game
In last night's adventure, the party was searching a goblin lair in hopes of finding and rescuing some human prisoners. They located the prisoners, who were guarded by a couple of big tough goblins and a handful of lesser ones. As the battle commences, one of the tough guards orders his lackeys to kill the prisoners unless the PCs put down their weapons. The goblins move to do so, but hold their actions, awaiting further orders. Cue Brenna the dryad, who has not yet spent her daily Charm spell: she throws it at the tough guard, who fails his save. Battle ceases, as the guard suddenly wonders why they're fighting his good friend, and the players have the good sense to suspend hostilities as well while this gets sorted out.
I determine that this goblin, though essentially Chaotic (I don't use formal alignments in my game, but I still find some concepts of it useful), does have some loyalty to his tribe and to his chieftain, even if only out of fear and habit. He dearly wants to grant Brenna's request to let the prisoners go with her, but he knows the chief will have his hide if he does. He gladly tells her why the goblins have them in the first place (captured pink-skins are slaves who labor for the tribe), but he's reluctant to simply let them go. After a bit of negotiation, he tells her she could talk to the chief, and if the chief agrees, he'll happily give her the slaves. He's so eager to help her out, and be rid of his terrible dilemma, that he shows her the location of the secret entrance to the lower level of the lair.
She asks him to accompany the party to meet with the chief, but he objects that the chief doesn't want him to leave his post. This isn't an excuse; the chief has given him a direct order, and he's loath to disregard it. The players decide not to try to force the issue. Since this is against the goblin's nature, but not overtly suicidal, it would have been possible, with a reasonably persuasive argument, to persuade him to go along. In the end, though, they let him stay, with the silver lining that, with him still on duty, the prisoners are unlikely to come to any harm in the PCs' absence because Brenna has made it clear that she wishes them to be treated well.
If they had persuaded him to go along, he would have tried to protect Brenna from harm, even from his fellow goblins, but would have been extremely reluctant to attack them otherwise, even on her orders. He probably would have hovered very near to her and fended off attacks with deadly force if nothing else would avail, but strongly resisted orders to help other PCs or kill goblins not directly threatening her. However, he would have had no problem at all in fighting other monsters in the caves. Indeed, the charmed goblin (from a different tribe) and hobgoblin whom she brought with her into this adventure have no qualms about fighting the goblins in this cave.
Other things a goblin might strenuously resist include entering a human settlement, riding a horse, and traveling when that awful blinding sun is in the sky. He'll offer alternatives, though. He's quite willing to skulk in the drainage culvert while the party takes care of their business in that nasty town. He'll promise to jog tirelessly to keep up with the mounted party, and plead with them to travel during the twilight hours around dawn and dusk. Push comes to shove, though, he'll acquiesce, but the party will have to endure his pleading and complaining.
When the tables are turned
Naturally, a DM ought to grant the same leniency to players whose characters find themselves charmed. Charmed PCs don't become mind-controlled pawns of the caster. They shouldn't be forced to attack their friends with deadly weapons simply because they have a new friend, but they'll use sufficient force to prevent their old friends from harming the new one. They'll probably prefer to exhaust non-lethal means first, though. (The reverse is not true; if the caster attacks the charmed PC's allies, the PC may plead or reason with the caster not to harm them, but won't harm the caster. The other PCs may be friends, but the caster is his Best Friend Ever, and while under the charm, everything the caster does, however unfortunate, is ultimately seen as justified.)