The main character is in prison or other cell-based captivity. Generally, they are entirely innocent, arrested on the flimsiest of excuses, possibly as a test subject for a mad scientist's experiment or some equally insane magical ritual.
You want to reveal this. But you don't want to sacrifice your main character, since you have, after all, devoted hours of screen time (or hundreds of pages). So, you introduce this trope. All you need here is a name and a face, and a few lines of backstory. The next day, something will happen to them.
Some villainous regimes will "seed" fake prisoners as Enhanced Interrogation Techniques; especially if they're trying to get some info. The Hero feels kinship to the other prisoner, and tells them what they would never tell their captors. If especially cruel, the warden will show off this betrayal later.
The Law of Conservation of Detail is responsible for this trope, as the story needs to elaborate on what is going to happen to the prisoners, but they can't use the main characters, so they need a Red Shirt to suffer instead.
- In From Dusk to Night, Dusky gets captured and imprisoned by the Leaf Cartel crime family. The next cell over has a stallion named Diligent Duster, who shares his sad story about how he ended up there and what the Cartel's already done to him. He mysteriously disappears just before Dusky can make her escape. Then he attacks Dusky, revealing that he's actually a Cartel member with a personal vendetta against her. He was just pretending to be a fellow prisoner as an overly complicated plan to mess with her psychologically before killing her.
- In I Against I, Me Against You, Rarity is captured by CT and his Insurrectionists and meets a fellow prisoner called Captain Erberle, striking a quick fellowship. She then explains to Erbele that her battle plan for dealing with CT will be to try the same trick she used against the Diamond Dogs in the episode A Dog and Pony Show (read: annoy the living hell out of CT through being the most insufferable prisoner ever). Unfortunately, when she actually tries to implement the plan, CT tortures Erberle to death in front of her as punishment. Rarity, horrified at what her actions have caused, stops completely.
- In Snow White and the Huntsman, the title character is in a cell near a young girl. After the witch gets through with her, she's an old hag.
- Subverted in Legend. Jack and his friends slide down the tunnel into a cell in Darkness's dungeons. They find a fellow prisoner: a fairy who worked for Darkness but betrayed him and was punished with imprisonment. While they're talking to him, one of Darkness's minions enters his cell and takes him out to be baked in a pie (no blackbirds, though). They eventually rescue him before he meets his fate.
- In BloodRayne II: Deliverance, Rayne just happen to be thrown in a cell next to the last living member of the Brimstone Society, and they are both scheduled to be hanged. Rayne, being a dhampir, survives the hanging, but her cellmate, being a normal human, doesn't.
- Played with in Thor: Ragnarok. After Thor has been Made a Slave of the Grandmaster, he is introduced to another captive (supposedly the Grandmaster's "cousin", although they look nothing alike, being of different species) who is promptly executed right next to Thor with the "melt-stick". However, the prisoners Thor later encounters in the actual holding area don't meet a similarly grisly end.
- TRON: Played in reverse. The opening scene is Sark, destroying a hapless blue warrior in a lightcycle match. The next scene is hopeless banking Program Crom being led to the holding cells, and Ram filling the poor fellow in on what he's in for. That's where we learn that the blue circuits mean they are User-believers, and the red means those who have sided with Master Control to overthrow humans. Later played straight (and elaborated on in the Novelization) when Crom is forced into a deathmatch with Flynn. note That's when it dawns on Flynn just how much trouble he's actually in.
- In Players of Gor, Tarl is imprisoned with Nim-Nim, a member of the Urt People. (Urts are giant rats about the size of a small pony. Urt People are humans who live within a herd of urts, are vaguely urt-like in appearance, and can "speak" urt.) Nim-Nim "helps" Tarl to escape, which turns out to be a joke on the part of the Big Bad, who planned the escape so that Tarl will be killed by the urt herd that Nim-Nim brings Tarl to. Nim-Nim excitedly rejoins the herd, but during his time in prison he has lost the herd scent, so he is descended upon and killed as an outsider—which is also Tarl's intended fate. Instead, Tarl kills an urt and uses its carcass to make the other urts think he's an urt, at least long enough to cross the herd and escape unscathed on the other side.
- Robert A. Heinlein's Have Space Suit – Will Travel. Kip is being held prisoner in the Wormfaces' base on Pluto. Two of his cellmates are the enemy's human minions who are going to be Rewarded as a Traitor Deserves because the Wormfaces decide that they have outlived their usefulness. First one of them is removed from the cell, then the other. The second one to go tells Kip their fate: to be cooked and eaten by the Wormfaces.
- In Airman, young and innocent protagonist Connor finds himself imprisoned in a subterranean penal labor camp. The harsh conditions are alleviated only by the presence of his cellmate, the blind composer Linus Wynter, who promises to help Connor through his new life. But on the second day, Connor returns to his cell to find it empty, with the warden saying only that Linus has been "released". Subverted when, years later, Connor learns that Linus actually was released on a clerical error, and the warden's usual idiom had for once been literal.
- In Starhunter, Dante's niece Percy gets arrested. They talk about how some of the prisoners are in "demand" by males, and she shrugs it off. Later, one of the other prisoners gets hauled off to be raped to drive the point home.
- Subverted in the Babylon 5 episode "Intersections in Real Time" in which Sheridan makes friends in breaks from being tortured with a Drazi fellow prisoner, who is then taken away to be executed. At the end of the episode, the interrogators bring the Drazi in again and reveal him to be one of them, as part of their general gloating demonstration to Sheridan of just how much they can mess with his mind.
- In The Book Of Genesis, innocent Joseph is put in a cell with two other prisoners. He decides to predict their futures by interpreting their dreams. He predicts that one prisoner will be pardoned by the Egyptians and one will be executed. He was correct on both accounts.
- Call of Cthulhu supplement Curse of the Chthonians, adventure "The City Without A Name". If the investigators are captured in the title location they will be put in individual prison cells. Each day one of them will be taken and fed to the immature chthonians.
- Dungeons & Dragons, module X4 Master of the Desert Nomads. If the bhuts capture the PC party they will chain all of them inside cells. Each night they will take away one of the prisoners (starting with any NPCs) and eat them.
- Castle Wolfenstein (1981). The introductory screens have information given by the Player Character's cellmate. It ends with the other prisoner screaming as he's being dragged away by the Nazis to be tortured.
- In the opening cutscene of The Suffering, the player character's cellmate (and...pretty much everyone else in the block) has the dubious honour of being the first victim of the monsters.
- In Tales of Legendia Shirles meets a friend in the cell adjacent to her. She soon gets dragged off. This turns out to be an inversion. She's the one who eventually gets used for an experiment.
- In The Legend of Zelda: A Link to the Past, Zelda mentions something happened to the other princesses, before Link saves her. Later subverted, as she finally finds out what happened to the other maidens, being banished to the Dark World and turned into a crystal, in order to break Ganon's seal.
- At the beginning of the fifth and final episode of Life Is Strange, "Polarized", Max wakes up tied up in a chair by the Big Bad. Depending on the player's choices Victoria might be there, drugged on the floor. Later on, after returning to the same situation via Time Travel, it's revealed Jefferson has killed her, and he's about to do the same to Max.