The other thing to consider is the amount of exception, characters are willing to make for their friends. A chaotic evil blackguard might sacrifice his friends last--unless his god demanded that particular sacrifice, in which case, he's probably sacrifice his friend anyway. On the other hand, he might readily betray some of his useful idiot companions for someone he thought was really his friend.
A paladin is unlikely to let a murderer escape just because he's a friend. He'd probably be kind to him, heal him and make sure that his manacles weren't too uncomfortable but he'd try to stop the murderer from getting away.
Charming the Lawful Neutral Pholtan Inquisitor is likely to get the charmer exactly nowhere. It makes no difference that he is a friend; the law is the law and it must be obeyed.
On the whole, charms are more effective on less strictly principled characters and less effective the stricter a character's principles are. I tend to think that good characters (and certain kinds of Lawful neutrals) are less vulnerable to charms as well since their principles aren't based on convenience and at least lawful good characters often feel an obligation to obey the spirit as well as the letter of their rules.
So, in addition to what the others have said, the personality of the character needs to be taken into account.