A villain held prisoner by the heroes continues to undermine their objectives through their toxic influence.
They have a tendency to spark disagreements between allies (up to and including the spread of a Hate Plague), use their captivity to shake the good guys' confidence in themselves or to make them feel uneasy by virtue of being closer around than they were as a free agent. Despite the heroes' best efforts, it's usually difficult to keep the villain in the dark about everything that's going on or prevent them from divining weak spots, so the longer they're held the more of a danger they pose, especially should they escape — meaning it's all but inevitable that they will.
Particularly dangerous cases may pose the threat of a hero's untimely corruption, if they start listening to the villain's cynical-but-persuasive advice, buying into their justifications for evil deeds, or being tempted by offers made in exchange for freedom.
Another common scenario is that the villain is held secretly, meaning that if word were to leak out, the captors run the risk of getting in serious trouble with their superiors, being deposed by their inferiors, or losing PR with the general populace.
All in all, it's a good reason to ignore Thou Shalt Not Kill, but the typical justification for keeping him around is that the villain has some kind of information the heroes need desperately enough for it to be worth the risk. In some cases, murder would be counterproductive or even impossible, if the captive is immortal.
May be a result of I Surrender, Suckers. Compare Pity the Kidnapper, a less subtle (usually) comedic version (which usually involves villains as captors rather than heroes), Might as Well Not Be in Prison at All, and Sealed Evil in a Can. Compare and Contrast Defiant Captive, who is more likely to be a heroic character refusing to submit to their captor. The inverted version often falls under Talking Your Way Out.
- Fullmetal Alchemist
- Envy, when they were trapped in a flask, was able to play on May Chang's fear for her clan and talk her into going back instead of taking them to Xing, which allowed Envy to escape and obtain a second Philosopher's Stone.
- Homunculus (the one later known as "Father") did much the same while trapped in his flask, using his knowledge of alchemy and the desperation of his captor to convince the Emperor of Xerxes to prepare the alchemic circle that destroyed the nation and gave Homunculus a body.
- Soul Eater's Medusa, for the brief time she was imprisoned, did absolutely nothing for Crona or Maka's peace of mind.
- The Thing Behind the Wall in Johnny the Homicidal Maniac. Nny has to keep it trapped by keeping the wall covered in fresh blood, which naturally does wonders for his already fragile sanity. Then the Thing gives two Styrofoam dolls in Nny's apartment sentience; The Plan was that they'd persuade him to commit suicide and thus release it.
- In Dark Empire, Luke Skywalker tries a version of this on the Reborn Emperor. He does some good, but is ultimately corrupted and his sister needed to talk him around.
- In Avatar: The Last Airbender – The Promise, deposed Fire Lord Ozai is visited in prison by his son and successor Zuko when the pressure of ruling begins to become too much for him. Ozai starts to advise Zuko on how a Fire Lord should rule. Zuko listening to him even the tiniest bit is a mistake he comes to regret.
- John Harrison (Khan) from Star Trek Into Darkness. After being locked in the Enterprise's brig, he almost immediately starts working on Kirk, needling him psychologically, feeding him information as it suited his purposes, and eventually ending up in an Enemy Mine situation with the Enterprise crew, rather than just remaining a prisoner.
- In U571, the German submarine prisoner continuously causes problems for the American crew, until he is killed.
- In Suicide Kings, a bunch of amateurs kidnap retired mafia boss Charlie Barrett for somewhat convoluted reasons. Needless to say, Charlie turns them all against each other in record time.
- Teaching Mrs. Tingle essentially uses this as a plot framework.
- In The Dark Knight the imprisoned Joker blows up the jail and works on a cop's psyche until he breaks out.
- Ruthless People at first, till it turns into A Match Made in Stockholm.
- In the film Southern Comfort, the National Guardsmen capture a one-armed French-only speaking local Cajun hunter and trapper, one of the Cajun's who were threatening them. They attempt to talk to him but he only speaks French. Nevertheless, just his presence is enough to poison the group.
- In The Avengers (2012), Loki is captured early on. Despite the team's best preventive efforts, it ends spectacularly poorly. It was so bad that in the end, everyone agrees that the best decision is to put a gag on him and send him back to Asgard.
- In Silence of the Lambs, Hannibal Lecter is a death row prisoner who manages to drive at least one other inmate to suicide, and whose understanding of the serial killer mind threatens to unhinge Clarice Starling.
- Following the vogue of recent movies, Silva in Skyfall gets captured as part of his plan to get revenge on M.
- In The Wheel of Time, the heroes captured no fewer than three Forsaken individually, and between them they covered just about all of the risks inherent to this trope. Asmodean was (sort of) on Rand's side, so the danger he posed was mostly just if word got out about who he really was, but you have to question the wisdom of keeping around a character like Semirhage, who was once captured by the forces of Light during the War of Power. By speech alone she terrified her guards so badly that they smuggled her out of captivity rather than face the tortures she threatened them with.
- The Silmarillion. Morgoth turns the Noldor against each other in Valinor once he was released from captivity. Later, Sauron ended up corrupting and destroying the Númenóreans after they captured him.
- In The Three Musketeers, Femme Fatale Milady de Winter pulls this by seducing her jailer when captured in England, twisting the guy so much that he becomes an assassin, attacking The Duke of Buckingham (also a Historical In-Joke, as this event really happened).
- In the Second Apocalypse series, Anasûrimbor Moënghus during the time he was a prisoner of Cnaiür urs Skiötha's tribe. He seduced Cnaiür's mother, convinced Cnaiür to kill his father, and talked Cnaiür into setting him free.
- Buffy the Vampire Slayer: In season four, Spike was often this to the Scoobies.
- Angel. In "Soulless", Angel has to bring back his evil alter ego Angelus. He's locked in a cage, so does his best to raise conflicts that have been simmering in Team Angel over the past season.
- Ben from Lost makes his first appearance as the captive "Henry Gale", and spends his entire time in captivity playing mind games with the survivors and attempting to turn them against one another.
- A Silurian warrior woman in series 5 of Doctor Who . She managed to convince one of the people guarding her to act on her worst impulses.
- Star Trek: The Next Generation: In Descent, a captive Borg actually succeeds in turning Data against the Federation. Albeit with some outside assistance from Lore, Data's literal Evil Twin.
- Game of Thrones had Jaime Lannister held captive by Robb Stark's army, sparking quite a few disagreements between Robb, Catelyn, and the Starks' bannermen. Not so much through any conscious effort to sow discord on his part, but they all end up divided about what to do with him, particularly after he murders Lord Karstark's son during an escape attempt.
- In Metal Gear Solid: Peace Walker the player can capture Zadornov and keep him in a cell in an attempt to convince him to join your cause. Ultimately, he breaks out numerous times and goes into hiding, forcing Big Boss to waste time locating and recapturing him each time. Each time he escapes, he's been released by Paz, who's using him to divert attention while she makes modifications to Metal Gear ZEKE.
- In Call of Duty: Black Ops II, Menendez (the game's big bad) gives up quite easily when the player catches up to him about 2/3rds of the way through the game. He's taken back to the USS Obama to be interrogated. You find out why he gave up so easily during the interrogation scene when the power goes out as a result of Menendez's followers launching an attack. Menendez mysteriously gets the drop on your buddy and then escapes; you find out later that your buddy is working with Menendez and in fact leads him to the ship's central computer where Menendez uploads a virus he had smuggled in. Whoops. Also, if you're dumb enough to spare him at the end of the game, and you didn't sacrifice Harper earlier, then he breaks out again and kills Woods.
- This is why the Brotherhood is trying to control The Darkness instead of destroying it in The Darkness 2. The Brotherhood captured The Darkness and guarded it for decades. And it kept talking to them until they eventually decided that destroying the world was the only way to save it.
- At the end of Diablo, the titular Big Bad is imprisoned in a crystal which is fused with the hero's body and mind, as Diablo simply cannot be killed. Cue the second game, where the hero is completely overtaken by Diablo who wears his body like a cloak. While this counts as Demonic Possession, it is implied that the process was gradual and did not rely solely on magic, but also at least partially on the personal influence and inherent malice of the demon. The same story also happened in the past with Tal Rasha and Baal.
- Cyndi from Penny and Aggie manipulated her mentally unstable captor into attempting suicide.
- This was Faden's entire schtick in Exiern. He was captured a couple of strips in, spent almost the entire time in captivity and still proved to be an effective villain.
- Elan's father Tarquin in The Order of the Stick attempts to invoke this while hanging above a long drop. Elan lets him fall instead.
"We could have some really intense interrogation scenes while I try to subtly manipulate you from inside my cell! What d'ya say?"
- The Joker on the Batman: The Animated Series episode "Mad Love", who was able to talk his psychiatrist Harlene Quinzel into becoming Harley Quinn while still in Arkham. It was implied early in the episode that he compromised the ones before her in a similar fashion. (Even scarier, the Joker had figured out how to escape from his cell weeks before he actually broke out of Arkham, but stuck around a little longer because he hadn't corrupted Harley yet.)
- Justice League: In "Injustice for All", Batman is captured by Lex Luthor's villainous team, and his words completely tear them apart from the inside. He bribes Ultra Humanite to betray the team to the League, seduces Cheetah into kissing him so they would assume she was his accomplice when they went back and reviewed the security footage to see why the plan failed, drives a wedge between team members and generally makes a nuisance of himself, all while restrained. Then, as the kicker, once everything is falling apart and the Joker uses the confusion to pay Batman a visit without supervision, Batman reveals in the climax that he could have escaped at any time but stuck around 'to keep an eye on' the Injustice Gang.