Basic Trope: The hero's friend has been brainwashed into a face heel turn, possibly even becoming a tragic monster. The hero must now fight the friend.
Straight: The hero fights the friend and manages to convince him that he's really a good guy successfully.
Exaggerated: The fight begins because the friend says something that is the tiniest bit out of character and everyone reacts - they aren't lost somewhere inside themselves, but their friends damn well think they are.
Downplayed: The hero fights a friend who had been brainwashed, but manages to remind him how much he enjoys fighting stronger people and turns him on the brainwasher.
Justified: The hero wants to save his friend, and can only help by fighting.
The villain fights a previously brainwashed hero to get him back under mind control.
After a heel realization the villain protagonist fights the hero antagonist
At the moment that the hero seems defeated, the friend leans in close and explains in a whisper that the mind control didn't really work, but the Big Bad has to think it did.
"Okay yes, your friend IS in here somewhere, but saying "I know you're in there" isn't going to get him out.
Double Subverted: After the friend tells the hero he's okay, the hero drops his guard, and the friend blasts him off, calling him a fool for actually believe what he said.
Parodied: The hero doesn't want to fight the 'friend' because he leant him bus fare a few years ago
Zig Zagged: The hero fights his friend, who is possessed by a demon. During the fight, the friend appears to go back to normal, but no one knows if the friend really is back to normal or if the demon is just acting like the hero's friend (until the hero lets his guard down).
Averted: The hero fails in redeeming his friend and ends up being defeated by the friend; alternately, the hero fails to redeem his friend and ends up killing him.
Enforced: The main villain wants the hero to kill his friend, and ensures that they end up fighting like this.
Hero: "I know you're in there somewhere!" Possessed friend: "Funny, the last guy I possessed's friend said that too!"
Invoked: Other friends or rivals of the hero says that only he can save his friend.
Exploited: The possessing entity who doesn't want anybody to know it's possessing somebody knows that it's been found out when the hero says "I know you're in there somewhere".
Defied: The hero refuses to fight his friend.
Discussed: "Please don't get possessed - I'd hate to fight a demonized version of you... Again!"
Conversed: "Amazing Girl has to fight one of her friends every five issues or so. And here we have it at the end of this issue, so next issue she'll probably try to talk her buddy down by reminding her of their friendship."
Deconstructed: As Genre Savvy as the Hero is, Reality Ensues and the Hero finds that it is going to take more that trying to remind his friend of the old days to make him normal.