They've caught a Body Snatcher or Symbiote, or are Sharing a Body, and you can tell, because they're obviously switching between minds repeatedly. Often accompanied by changes in manner, posture, voice, or eye color or shape. Those not aware of the circumstances and not sufficiently well acquainted with the originally one-minded character may not notice the change, leading to much opportunity for hilarity to ensue.
Having arguments with yourself is strictly optional, but again, hilarious.
Examples:
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Anime and Manga
- In Black Butler, Alois' soul ends up in Ciel's body and both souls dispute its control. You can see — quite easily — that Alois is in charge because of the (not subtle) change in attitude and because the eye seal disappears.
- Taken to extremes with Launch in Dragon Ball, whose body undergoes a physical change when she switches personalities.
- In Fullmetal Alchemist, Ling is possessed by Greed, and eventually they start taking turns being in control. You can tell who's who because Ling has his Eyes Always Shut, while Greed opens them; they also have different voice actors in Brotherhood.
- In Yu-Gi-Oh!, Yugi and Yami/Atem do this in their duel against Pegasus, who can read minds, but only one at a time. They flip personalities so Pegasus can't read their strategies.
Comic Books
- Batman: Most Post-Crisis interpretations of Two-Face employ this, where the "good" Harvey Dent sometimes externalizes his struggles with the "evil" Two-Face.
Fan Works
- Abraxas (Hrodvitnon): Early on in this Godzilla MonsterVerse fanfiction, the two halves of Monster X (San and Vivienne Graham) do a downplayed form of this; in their first hybridized form, before San's head gains its own mouth, Viv and San take turns with control of Vivienne's mouth when they both want to physically speak. Later in the story, this trope becomes played somewhat straighter in Monster X's single-headed final form, with San and Vivienne's personalities performing this trope when speaking independently.
- All Assorted Animorphs AUs: In "What if Tom was infested by a member of the Yeerk Peace Movement?", Tom and Aftran have a symbiotic relationship where Tom lets Aftran take control of his body with permission, and she speaks in a different tone of voice. It's mentioned that they talk to each other out loud, which looks incredibly weird to everyone else.
- In The Bridge, Monster X has a Split Personality: the Noble Demon Monster X and the psychopathic, rage filled Kaizer Ghidorah with the split having been triggered by the death of his wife Controller 011. When in Kaiju form, it's obvious who's in charge as Monster X goes from a skeletal armored humanoid dragon to a massive three headed, quadrupedal dragon. In human form, it's a little more subdued: their skin and hair changes from white to black and his eyes become red and black. Kaizer also has his mouth exposed rather than wearing a mask to cover it.
- Infinity Train: Knight of the Orange Lily: Specter has obtained a Split Personality that names themselves "Easter" — a corruption of a phrase Earth-as-Specter, later revealed to be a lightning bolt that accidentally struck him down. Easter makes themselves well known with a playful smirk and electricity crackling in their fingers compared to Specter doing his best to keep his sanity intact.
- In the Pony POV Series:
- Pinkie Pie's Pinkamena personality has straight hair, though unlike many fanworks she's not psychopathic and a more realistic split personality: she was created to handle negative emotions Pinkie didn't want to feel. It's unclear if this effect holds true with Pinkie's other three personalities, as only Pinkamena is ever seen in control other than her before she performs a Split-Personality Merge.
- Fluttershy is Sharing a Body with Fluttercruel, a second spirit sharing her body born from Discord's magic and, in a sense, the daughter of her and Discord (though not intentionally on Discord's part or consensually by Fluttershy). The two have vastly different personalities, so their body language obviously changes with whoever is in the driver's seat. Eventually, Celestia grants them the ability to change Fluttershy's appearance when they tag out, complete with Fluttercruel's Cutie Mark.
Film — Live-Action
- Dave Made a Maze: The cardboard duplicate of Brynn flips back-and-forth between cheerful and asking for high-fives, to intimidating and demonic-sounding. Harry wonders if the thing actually is Brynn, and she's attempting to fight off the maze's influence.
- In All of Me, Edwina Cutwater (Lily Tomlin) is dead and her spirit is sharing a body with Roger Cobb (Steve Martin). Martin snapping back and forth between the Edwina and Roger personas is fairly impressive.
- In Split and Glass (2019), there are scenes where Kevin Wendell Crumb, who has 24 personalities, flips through half a dozen of personalities. Split has the several good personalities beg Casey not to shoot them after Kevin asks her to before the Horde retakes control. Glass has a dozen alters briefly say some final words as Kevin lies dying from a sniper bullet with Kevin managing to hold onto at the very end.
Literature
- Demons in the Chalion series. Those with second sight can see who is in control of a body.
- Odetta Holmes & Detta Walker in The Dark Tower.
- Altogether Andrews from Discworld, first introduced in The Truth.
- Also Agnes Nitt and her alternate personality, Perdita. No physical change, but the personality change is very, very distinct.
- In Early Riser, Aurora and Toccata have a psychosomatic bad eye that switches when they switch personalities.
- Gollum/Sméagol in The Lord of the Rings, especially in the movie adaptations.
- In The Stormlight Archive, Shallan's Split Personality uses her Lightweaver powers to clearly distinguish the three of them, with Shallan keeping her natural red hair and Veil and Radiant using Lightweaving to appear brunette and blonde. They also have distinct tastes in fashion and different ways of covering their left hands: Shallan uses a full sleeve, Veil uses a glove, and Radiant doesn't even bother.
Live-Action TV
- Doctor Who:
- "The Satan Pit": Toby. When he's possessed, his eyes turn red, his body is covered in arcane writing, and he shouts imprecations in a gravelly voice. It turns out, however, that the times when he appears and acts normally are actually a ruse.
- The Doctor/Mr. Clever in "Nightmare in Silver" like to switch which side of the body is in use.
- The Stargate-verse has several examples:
- The Tok'ra symbiotes and their hosts from Stargate SG-1. The symbiote taking control is revealed by an altered voice and temporarily glowing eyes.
- In the SG-1 episode "Lifeboat", a quite large number of consciousnesses get downloaded into Daniel, and he sort of cycles through them.
- In the Stargate Atlantis episode "Duet", Cadman and McKay share a body as the result of an accident with a Wraith transporter beam, and spend some time fighting over it.
- Stargate Universe: Episode "Hope". Two more consciousnesses got uploaded into Chloe's mind.
- There's a Star Trek: Deep Space Nine episode where other characters voluntarily share their bodies with the personalities of previous Dax hosts. Quark is very uncomfortable with the personality he's hosting and keeps emerging to complain.
Video Games
- Eternal Darkness: One of the gods, Xel'lotath, has two personalities that act like this.
- In Inscryption, Leshy represents his characters with masks. The Trapper and Trader actually use the same mask flipped upside down; the Trapper's beard and wrinkled head become the hair and scarf of the Trader and it's implied they are the same person. Act 2 confirms this when the Trapper, now a full character, rotates their head upside down to become the Trader.
- MARDEK:
- The Hero shares his body with the soul of an alien mage called Rohoph. You can tell instantly when Rohoph is talking because Mardek's eyes glow white and his speech pattern (and font) changes completely.
- Legion, a robot animated by dark magic and containing four different souls. One's an Evil Overlord, one's completely insane, one's a mopey, depressed guy named Marv- er, Martin, and the last one, a shaman, is the Only Sane Man. Their text color changes depending on which one's in control at the moment.
- Persona 4: Arena Ultimax has Sho Minazuki, comprised of Sho, a short-tempered boy who constantly makes puns and who has no Persona; and Minazuki, cold and concerned with absolutely nothing other than fulfilling Sho's dreams who does possess a Persona. The two flip back and forth a lot, with the change being marked by their body going completely limp for a moment. Minazuki even occasionally takes over while Sho is still in mid-rant. This odd arrangement came about when Sho was implanted with a Plume of Dusk, the same sort of Heart Drive that Aigis and Labrys have, as part of Ikutsuki's experiments to artificially create a Persona in a non-Persona-user.
- BlazBlue: Platinum the Trinity has two souls in one body. Luna is a rude and brash girl, while Sena is a polite, awkward and softly-spoken young boy. They flip often enough to be on a sentence-by-sentence basis at times, which is most clearly indicated by the marked change in voice and mannerisms. Each one taking control in turn can also result in Platinum's facial expression changing several times in a row. There's also Trinity Glassfille, whose soul is bound to their magic wand and only speaks through them very rarely — when she does, the voice changes again and Platinum's Exotic Eye Designs disappear.
- In Vampire: The Masquerade - Bloodlines, you eventually discover that the Voerman sisters, Therese and Jeanette, are actually the split personalities of a single Malkavian vampire. At the end of their quest line, their rivalry comes to a head, and as you talk with Ms. Voerman she rapidly switches between talking like Therese and Jeanette as the two argue.
Visual Novels
- Danganronpa: Trigger Happy Havoc: The Mastermind, aka Junko Enoshima, flips between at least half a dozen personalities. She claims she gets bored easily, we claim that she's completely insane.
Webcomics
- Lucrezia and Agatha in Girl Genius. No outward signs of transition, but obviously different personalities.
Web Original
- In Red vs. Blue, when O'Malley possesses Doc, their body frequently switches personality on a sentence by sentence basis, this is shown by voice and mannerism changes and camera angle changes. The two characters actually hold conversations with each other, or will cut in on what the other is saying to make some kind of wise-crack or amendment. For the most part O'Malley is in control of what they do, however. All the characters are pretty much aware of this as well, and since it's a comedy no one really cares except when O'Malley is trying to kill them and take over the world.
Western Animation
- Blitzwing from Transformers: Animated has three personalities known as Icy, Hothead and Random. Which is in charge at that moment is indicated by his whole head spinning so one of his three faces is visible.
