I think the best example that happened in one of my campaigns was the following:
This tale begins with the story of Siabrey (a female half-siabre fighter) and Lucius, a human fighter/sorcerer. Siabrey was a PC, Lucius an NPC, and they met when she saved his life. Long story short, they fell in love, and eventually they got married. Except there was one problem. :\
Lucius' mother was an evil sorceress bent on world domination, capable of gating in an army of demons and fully intent on destroying anyone who opposed her. The party, including both Siabrey and Lucius, were trying to stop her at every point and turn, and the mother naturally wanted some 'alone-time' with her son to convert him, (by magical force, of course) to her evil ways.
Needless to say, she set a trap, which the unfortunate party fell into (they learned the hard way that when guards come to take you to a 'secure location' that you need to check for knockout gas along the way). Put into a forcecage, they watched powerlessly as Lucius was kidnapped by his mother and whisked away.
There was no time to cry for Siabrey, though, for the evil sorceress' armies were planning to attack the city they were in! After the party freed themselves, Siabrey found herself the only one capable of organizing the city's defense, a task she took to out of despair. When finally the great battle occurred, she was on the frontline, desperately holding the wall against all comers.
Until there was a flash behind, her, as Lucius teleported in. He was now demonic, evil beyond compare, and under the utter control of his demonic mother. His orders were simple; kill all who tried to stand in the path of his mother, of which Siabrey was on the top of the list. Now I (as the DM in this menagerie campaign) expected something tense ot happen, the choice in Siabrey's player's mind on whether to fight the character's beloved or not. As Siabrey had tended to be a rather rash and bold character, I expected some kind of combat, save she'd hold back at the end to try to save him.
Instead, (I think for one of the first times in the campaign), Siabrey sheathed her swords, and walked to her husband, even as he drew his blade and began buffing incantations. She tried to talk to him, calm him, asking him if he remembered her. He didn't, and only yelled that she needed to die. As he brought his blade down, she didn't draw hers, instead only uttering, "Even if you kill me, I shall always love you."
Fortunately for this story, the party's bard (variant style) had some sense, and used her magic to paralyze Lucius just before he could strike his wife with the first blow. The young man was held in such state long enough the party could rush him to the local high priest of Hieroneous, who helped break him of the curse.
I was really impressed with the player's empathy to her character and willingness, for the sake of roleplay, to possibly lose her character (she had no idea the bard was going to help, or if the help was going to work). Needless to say, there was a major roleplaying XP award at the end of that session!
