A point through example:
The most important matter to remember, I think, about charm person is that although the spell has changed the way the target thinks about the caster, it has not changed who they are or what they are doing or why.
I don't like using my games as an example because YMMV but:
Way back in my campaign, the characters were up against an orc warlord who, with the help of an Arcanloth and other denizens from the lower planes, managed to unite the local orc tribes into an army set on conquest (a little cliche I know but it was fun). During an encounter with a scouting party, one player (it was R actually) seperated her character from the main battle and cast charm person on a lone orc before he entered the fray.
What happened was, the orc came to her and when she explained his friends were under attack he wanted to go help but she said not to because of the danger. I ruled that the orc would take his "best friends" advice. Then I thought about this orc: a typical orc warrior raised in a typical orc society currently serving in a typical orc army.
The next thing the player knows, her character is being dragged by this orc across the country side back to the main camp to report. All along the way she kept asking him questions and he wouldn't really answer them, only telling her bits and pieces.
I decided all this on the basis that even though the orc was charmed, he wasn't fundamentally changed in his principles. He wasn't going to give up to the PC's, and he wasn't going to betray his warlord chief either, and even though his "best friend" was asking questions he wasn't going to spill the beans because that was a security risk. That last decision was also made with the idea that your typical orc's views on women probably aren't the same as a human's and that this orc under the charm person viewed her not as a friend but as a favoured piece of property (which is why he was bringing her back, to be his new best wife/trophy).
I hope that illustrates my point; that charm person is not a spell that changes the targets mind radically and therefore everything they were before is still an important factor in their decisions.
Interestingly enough, that orc started out as a meaningless scout and through a bizarre series of events including the charm person, a serious behavour modification spell that we no longer use, a pixie memory loss arrow, and being raised by a Satyr, has become a serious NPC and the husband to R's character.
Weird huh?
J from Three Haligonians