In a recent game, I was given a mission to kill a specific NPC. We got into a situation where there was a fight with a summoner in a tavern, and that NPC was watching (and not participating) in the fight. I wanted to Charm Person him, and tell him something to the effect of "Save me, that eidolon will certainly kill me unless you fight him off!"
For reference, the text of Charm Person (emphasis added):
This charm makes a humanoid creature regard you as its trusted friend and ally (treat the target's attitude as friendly). If the creature is currently being threatened or attacked by you or your allies, however, it receives a +5 bonus on its saving throw.
The spell does not enable you to control the charmed person as if it were an automaton, but it perceives your words and actions in the most favorable way. You can try to give the subject orders, but you must win an opposed Charisma check to convince it to do anything it wouldn't ordinarily do. (Retries are not allowed.) An affected creature never obeys suicidal or obviously harmful orders, but it might be convinced that something very dangerous is worth doing. Any act by you or your apparent allies that threatens the charmed person breaks the spell. You must speak the person's language to communicate your commands, or else be good at pantomiming.
The other players tried to convince me that I couldn't use Charm Person that way. By my reading of RAW, it explicitly allows me to compel them to make "very dangerous" actions as long as A) it's worth doing (in this case, to save the life of a close friend) B) it's not obviously harmful, and C) I win the charisma check. My impression of "obviously harmful" is something that will assuredly cause harm (so no "put your hand on that hot stove"), but since combat is something from which you easily could escape unscathed, it seems "very dangerous" rather than "obviously harmful".
The combat ended before I had a chance, so the GM didn't get to make a ruling, but for future reference I'd like to know - can an otherwise neutral NPC ever be charmed into joining a fight?
Sure, if that's the sort of thing the NPC would do for a friend.