When a villain is sufficiently sympathetic with the audience, he has a tendency to do a Heel-Face Turn and this coincides with a tendency to do Face Heel Turns when it's realized that he really worked better as a villain. The result is a further temporary Heel Face Turns because they are uncomfortable with their audience Rooting for the Empire.
This phenomenon works the other way, as well. The Hero loses perspective and becomes a Well-Intentioned Extremist, and then comes back from the edge again. He's done it before, and it worked well (narratively) that first time — why not do it again?
The long-term result is the same either way — the character in question will switch sides often enough that, in the long run, he doesn't have a side. This is what makes a Heel Face Revolving Door (or Face Heel Revolving Door, depending on which side the character starts out on).
If it happens to a popular or well-developed character, the fans will stick with them; but this will, by necessity, drag the morality of the series to one of the gray-scales.
This is common in Comic Books, media using the Fleeting Demographic Rule, and collaborative media written by fans Running the Asylum. It's easier with characters who have what is initially a Never Live It Down moment or a Remember When You Blew Up a Sun? in their past.
Enemy Mine can facilitate this. It's often understood that switching sides through Enemy Mine won't create a permanent change of allegiance but if a sufficiently high percentage of a character's appearances are Enemy Mine, this is one of the possible implications.
Someone who has been through the revolving door too many times may turn into the Wild Card. If mostly on the side of good, despite having decidedly nonheroic intentions, may be a Nominal Hero.
Compare Chronic Backstabbing Disorder (which is more about switching between grey groups than white-to-black or visa versa), Wild Card (where the character isn't strongly on anybody's side ever), Unscrupulous Hero (a character who is unambiguously on the heroic side), and the Double Reverse Quadruple Agent (he never changes sides, but no one is sure what side he's really on).
Almost all important characters in the second and third movie do this.
Indy's partner in Indiana Jones and the Kingdom of the Crystal Skull. Fairly early in the movie, he's with Indy. Then, we discover he's working for the bad guys, then about halfway through the movie, he is still concerned about Indy's health, but is still working for the villains. A little more than halfway through the movie, he says he's a double agent, then at the end, he reveals he was lying about being a double agent, but he seems repentant of his actions when he choses to stay in the collapsing temple while Indy's friend and family escape. In all likelihood, he was just continually picking the paths that would lead to his one true goal: knowledge and treasure. His "repentance" was just the realization that the alien ship contained all he ever sought, and he no longer had any need or desire to battle that stubborn old adventurer.
Also the same case for The shrek version of Rumpelstiltskin between 3 and 4.
Godzilla goes through this pretty much every other movie. He was undoubtedly an unstoppable menace in the first few installments, but as the original series continued to drag on, it slowly turned him from Villain to Anti-Hero to outright Hero. This was reset in Godzilla 1985, where he was a villainous monster once again, and ever since he has been more consistent as either a Villain or Anti Hero.
Tiffany from the last two Childs Play films. She does a Death Equals Redemption at the end of Bride Of Chucky, only to come back in Seed Of Chucky back in love with the equally, if not more, psychotic Chucky. However, upon learning that they have a child, Glen, Tiffany tries to give up killing to set a better example. Only she has a number of slip-ups and murders twice (and possibly more due to a Time Skip). She justifies these as "Rome wasn't built in a day", and that one of the people she killed had it coming.
In Despicable Me 2, Dr. Nefario joins up with El Macho so he can do evil deeds once again, but when he learns that El Macho is holding Lucy hostage as well as learned of the evil minion El Macho had sicced on the girls, he returns to Gru.
Noob is set in a MMORPG with established factions and changing an avatar's faction is possible. Starting Season 4 / the fourth novel, the player behind Gaea is technically constantly switching sides: she performed a Face Heel Turn from her guildmate's point of view with her main avatar and her secondary one is still in the same faction as said guildmates, who happen to be understaffed enough to let her tag along. She however tends to work only for her own advancement, leading to her using her main character to break her secondary character and her early guilmates out of a prison cell in the fourth novel.
This can be literal if a wrestler or tag team is employed by more than one company at a time, leaving one building as heel and coming into another face a day later. Common in the indies.
And then there was the even more disjointed era of 1999-2000, due to the nonsensical, incoherent booking techniques of Kevin Nash and, later Vince Russo and Ed Ferrara. Anyone was fair game for a turn one way or the other, even the most popular of fan favorites such as Goldberg and Sting, with little or no warning, and, in the case of lesser stars, sometimes little or no explanation. And that's not even getting into the countless fakeouts.
In fact it's pretty rare, especially in the last few decades, for any notable wrestler to not go through at least a couple of Heel Face Turns and Face Heel Turns over the course of his career. The main exceptions in modern professional wrestling are people whose careers simply didn't last long enough to turn from heel to face or vice versa.
Averted by Ricky Steamboat, who never once turned heel during his career, due to being such a natural face. To give an idea of how good of a face he was (or how horrible of a heel he would have been), Steamboat's real name is Richard Blood, and yet he had to use a different name because that was a heel's name.
The name was used by Tito Santana early in his career, and he is also someone who was a Face his entire career.
Supposedly during his run in the WWF in the very early '90s he wanted to do a heel turn. He was told that even if he went out to the ring and cut Hulk Hogan's arms off with a chainsaw the fans wouldn't buy it.
Also averted by Rey Mysterio Jr. Has never been a heel. He may have been sort of considered a heel, sort of, in the WCW faction "The Filthy Animals" but even then the stable was more of a tweener thing, and he was not at that long before becoming a full fledged face again.
Kane is probably unique in that the booking team tries to make him a major-league wrestling monster heel every couple of years but as soon as they stop paying attention to him (i.e. as soon as he's not running around setting announcers on fire or suchlike), the fans start cheering for him again. May have something to do with his Woobie backstory.
It's gotten to the point that Kane will go through the revolving door two or three times a year, if not more. When asked Kane's alignment, the best answer is simply "Kane."
Happens often with wrestlers who are too likeable or dislikeable for any turn to take. No matter how heroic he acts, a lot of fans can't bring themselves to cheer Randy Orton. Eddie Guerrero had to virtually murder Rey Mysterio for the fans to buy him as a heel and even then, as soon as he wasn't acting like a complete maniac, they started cheering for him again. Ditto "Stone Cold" Steve Austin's attempted heel turns after he became "Stone Cold".
Ended up getting a variation in Orton's case; he's recently become cheered as a face-by-default, thanks to feuding with Cody Rhodes and Ted DiBiase Jr., despite still showing heeling tendencies. Turns out he didn't need heroics to be a face, he just needed opponents nobody liked.
If you're a TNA wrestler prepare to flip-flop more than a goldfish surrounded by water and a broken fishbowl.
See: Mr. Anderson.
Not even Jeff Hardy was immune. And at the time he could have been second in-line for being the Ricky Steamboat of his generation, having only been heel once.
Scott Steiner. Starting at the second half of his WCW run, the only way to know whether he was a face or heel was the opponent he was going to face next. He's not a traditional Tweener though, because he was supposed to go through Face Heel Turns and Heel Face Turns, it's just that nothing changed between them, so nobody really noticed.
The Big Show is notable for turning about once a year. Being the largest guy on the roster means that he can play the role of The Brute against guys like Kane and The Undertaker, and just demolish the plucky babyfaces like Rey Mysterio Jr. and Kofi Kingston. However, the guy has great comedic timing and plays the role of the Gentle Giant so well that bookers will eventually turn him. He will then proceed to be great friends with the Rey Mysterio's of the world.
One infamous occasion happened in early 2003. Vince McMahon appeared on Raw to override the heel GM Eric Bischoff but then a few weeks later was involved in an angle with Hulk Hogan as a heel leading into WrestleMania XIX. So he was effectively a face on Raw and a heel on Smackdown.
Another odd case with Vince occurred in 1999, where he was a heel going up against "Stone Cold" Steve Austin and the Ministry Of Darkness, led by an also-heel Undertaker, heading into WrestleMania XV. He soon turned face when Undertaker started threatening his family, but again turned heel when he was revealed to be the Higher Power behind the Ministry, and remained there until he was forced to "leave" after Fully Loaded. Again, Vince turned face when he returned a few weeks later to oppose main event heel Triple H and stayed face until WrestleMania 2000, when he aligned himself with The Game.
In late 2010, Nikki Bella turned heel in an angle on WWE NXT but was a face whenever she appeared on Raw and Smackdown. When that season of NXT was over she went back to being a face for a couple of months before she and her sister turned heel properly.
Chyna had this particularly bad during this time. When she was on her own she stood up for women's rights. When she was with DX she would punch guys in the groin to score a cheep win for Triple H. Sometimes she would switch from one role to the other multiple times on the same show. Once she won the Intercontinental title however, she was quietly phased away from Triple H and stayed face.
Chris Jericho doesn't really do this all that often, but he's so good at playing heel and face, it's no wonder he's been both multiple times. Including his most recent comeback, where he came back to face pops and managed to draw out heel heat by doing next to nothing.
Alicia Fox turned face in 2012 but roughly a year later started appearing as both a face and a heel depending on the whims of the writers. A fan actually asked her on Twitter which one she was and she responded "both".
In DBSK's music video for BeforeU Go had a friend of Yunho and Changmin do this. First, he joined the police force with Yunho and Changmin. Then for an unknown reason, he was The Mole for the mafia. After he accidentally shot Yunho and was caught by Changmin, he decided to quit. A year later, he forced to kill Changmin when the mafia held his sick sister hostage. By the time things were over and after Yunho and Changmin found out the truth, they could never fully trust him again.
It's hard to keep track of anybody's moral alignment in Narbonic, probably because nearly everyone is either insane (and varies the degree of evil and their motives beyond the speed of logic) or just there for the paycheck.
Tarvek Sturmvoraus in Girl Genius alternates between helpful, heroic, not-entirely-trusted sidekick and toadying, evil, certainly-not-trusted minion when heroine Agatha Heterodyne and villain The Other start grappling for control of Agatha's body. He makes the switch every few pages, eventually caught red-handed by the Other. Even this doesn't stop him from serving his own purposes, eventually betraying everybody. Absolutely Everybody. Including his own father and his treacherous sister. And it seems that his goal is to use Agatha (or a lookalike), his claim to a royal ancestor, and a story out of mythology and opera to not only rule Europe, but do it to cheering crowds.
Now it appears it MAY be a more sincere Heel-Face Turn and he genuinely wants to help Agatha. For now.
Gil's manservant Wooster may have changed allegiences from his British spymasters to Gil and then possibly from Gil to Agatha—or he never abandoned his first loyalties. Given the way Sparks influence non-Sparks, it's not entirely clear.
Oasis from Sluggy Freelance. At first she was an (admittedly sympathetic) Yandere villain who was obsessed with Torg and would kill anyone or anything to be with him. After Torg promised to marry her, though, she shifted into a (admittedly anti-heroic) good guy, helping to take down Hereti Corp and protect the town of Podunkton. However, after her Mentoris killed in front of her and Zoe rekindles her jealous streak, she's off on another Roaring Rampage of Revenge.
Prior to Dr. Schlock'stakeover of Hertit-Corp, he tended to change sides quite often, although it was almost always based entirely on who was pointing a gun at his head at the time.
Nicolae of Gaia Online tends to do whatever gives him the most benefit at the moment, be it con artistry, theft, assisting players in zOMG! or accepting a hit job on Gino Gambino, which he then takes every opportunity to sabotage.
In Sinfest, Fuchsia bounced back and forth between crushing on Criminy and actively helping Blue and the Devil antagonize Slick and Monique until her character development led her to pretty much stop being evil.
lonelygirl15: How many times did Gemma either change sides or turn out not to have changed sides after all? In a relatively small amount of appearances, at that? Granted, this doesn't seem to have been a result of lazy or inconsistent writing - the woman just couldn't commit.
There's also Sarah. In her first appearance, she appeared to be a villain when she held the heroes at gunpoint; but then she sided with the heroes; but then she got brainwashed by the villains; but then the heroes rescued her from the brainwashing; but then it turned out she'd actually been working for the villains all along; but no, wait, she was actually being blackmailed that time, she's really on the heroes' side; and now, the sequel series lg15: the Resistance has her once more turning out to really, actually, honest to God, this time we mean it, have been on the villains' side all along.
Making it worse is that a video was actually put up solely to explain how various actions we'd seen Sarah taking over the past weeks were signs of her villainy. It couldn't be more obvious that all the footage in it is from the spinoff, for the simple reason that there's no freaking way to incorporate the majority of her scenes in the original show into the new story.
Cale from Darwin's Soldiers switches sides no less than four times, playing for almost every faction (experiments, terrorists, scientists, Dragnostorm) at some point. This is lampshaded by Dr. Shelton.
In Pavlov's Checkmate, Cale finally gets over this, refusing to switch sides when the antagonist gives him the offer.
Everyone in the YWC (Youtube Wrestling Community) seems to switch sides so much that they seem to live inside this door.
Definitely occurs, to pretty much everybody, in the Prolecto Series. Starts with the standard virus induced Face-Heel Turn, then most of the Succubi switch to Faces, then a few switch to Well-Intentioned Extremist, then they switch back, then one of the Faces goes Heel...Then some people who stayed Heel go face. A couple actually qualify as Hazy Feel Turns, actually.
A few examples in The Questport Chronicles, but most noticeably the Lord of Angels and Demons, who switches sides at the drop of a hat. He's never trulyevil, but he oscillates between being mildly useful and incredibly annoying.
Red vs. Blue: Agent Washington has had a few trips through the revolving door. He starts off firmly on the side of the Director of Project Freelancer, defending and trusting him more than just about anybody else. When he learns the truth about the origins of the AIs the agents use, he secretly begins working against Project Freelancer. Then, after getting sent to prison due to the mysterious disappearance of critical evidence that would prove he was on the side of good (thanks to a certain Blue's idiocy), he agrees to work with the Meta (the villain of season 6) to get the evidence back. When the Meta betrays him, Wash realizes his mistake and works with the Reds and Blues against the Meta. So far, the last Heel-Face Turn seems to be sticking... mostly because he realized that even after all he did, the Reds and Blues still forgave him.