Sometimes, insanity isn't easily noticeable in a character. Sometimes the Mask of Sanity remains fixed very carefully in place, to the point that the character really could seem just as sane and rational as anyone else on the face of the Earth...
...right up until they start singing.
Perhaps they're suffering a very nasty case of Sanity Slippage; perhaps they were demented all along and this is just the final reveal; or if you're really unlucky, perhaps their lunacy is even worse than anticipated. Whatever the case, a character's insanity is firmly demonstrated via an inappropriately-timed song - or at the very least, a very spirited attempt at it. As further proof of madness, chances are that the character's enthusiasm will far outweigh their ability to actually carry a tune.
May possibly overlap with Soundtrack Dissonance if the choice of music really doesn't match the mood of the scene, Ironic Nursery Rhyme for a darkly childlike edge to the madness, Madness Mantra if the singer gets stuck on repeating the lyrics, or Hell Is That Noise if the song is played for horror.
Please note that this is not the same thing as a Sanity Slippage Song or a Villain Song: this trope does not involve musical numbers, is rarely if ever accompanied by background music, and most commonly takes place in a non-musical work - hence why the use of singing is considered so out-of-the-ordinary by both the audience and the other characters in the story.
- Big Finish Doctor Who:
- In Jubilee, the demonstrably insane Miriam Rochester idly hums a few bars of "God Save The Queen" while getting ready to usurp her husband's throne - concluding by singing "God Save Me." For added lunacy, she barely reacts to the fact that her lover's severed head has just been dumped on the dressing table in front of her.
- The Jester's earliest scenes in Axis Of Insanity feature him chanting "We're Off To See The Wizard" and "Doctor Foster" as he leads the Doctor deeper into the Axis - and a trap. Also he's very up front about the fact that spending an interminable amount of time stuck in a "Groundhog Day" Loop within the Axis has driven him a little loopy.
- Night Thoughts plays this for tragedy: during her audio diary, Maude reveals the circumstances that led up to her daughter Eadie's surgically-assisted Mercy Kill - and the fact that the whole thing was completely pointless. Growing less and less lucid as the recording continues, the now-suicidal Maude concludes by singing Eadie's favorite nursery rhyme - "Oranges And Lemons" - breaking down in tears as she does so. In the present, Eadie's reanimated form can be heard whistling the same tune while hunting down the research team.
- Over the course of Enemy Of The Daleks, Lieutenant Beth Stokes has been clinging to sanity by a very narrow thread as the Daleks' winning streak begins hammering on her Trauma Button, until the prospect of being captured alive - and having to relive the horror of her childhood enslavement all over again - causes her to completely break down. Ace finds her collapsed in the kitchen, borderline catatonic and whimpering a lullaby.
- In Captain Britain: A Crooked World, the main-universe counterpart of Mad Jim Jaspers spends most of his tenure appearing relatively sane; he's got the same powers and goals, but prefers to play at being prime minister for the time being. However, as his abilities grow, so does his insanity, eventually resulting in a scene in which he gleefully unleashes his true power on Downing Street and everyone in it, ultimately transforming the Vixen into a fox while singing "A Four-Legged Friend."
- During Hellblazer: Son Of Man, Harry Cooper is introduced inanely singing "Roll Out The Barrel." However, rather than making him creepy, it's meant as an indication of just how far he's fallen: he's seriously ill, looks as though he's swallowed a zeppelin, and oblivious to the world around him; by this point, singing the tunes of his childhood is literally the only thing he can do. He's actually pregnant with the Antichrist, courtesy of the demon animating his son's body, and it's left ambiguous as to whether Harry's current behavior is due to the biological effects of playing host to a demon fetus or simply due to the trauma of being cut open and raped by his own son.
- Rajijah Juice sometimes has this as a side-effect in Tintin, the main effect being Cloud Cuckoo Lander-style madness. Having observed the random singing in other victims, Tintin himself mimics this in Tintin: The Blue Lotus while pretending to be under the drug's influence, leaving the villain's office while singing nonsense at the top of his lungs.
- Played for laughs in the Bioshock Infinite fanfic After The Burial. Here, Elizabeth goes completely out of her mind due to a combination of alcohol, drugs and long-term depression, then goes on an increasingly destructive rampage around Rapture. At one point, she teleports a huge chunk of modern-day Paris into Dionysus Park, resulting in widespread destruction, violence and confusion, and is seen merrily drifting through the carnage while singing La Vie En Rose.
- 2001: A Space Odyssey provides one of the most famous examples of this: having gone violently insane due to a Logic Bomb, HAL-9000 is given the electronic equivalent of a lobotomy and forcibly deactivated, resulting in him singing "Daisy Bell" as his mind slowly goes down the drain.
- In Aguirre, the Wrath of God, the eponymous main character's second-in-command has a habit of singing monotonously to himself just before doing something nasty. As such, it's his singsong muttering of "la, la, la, la" as he destroys one of the rafts that first reveals Perucho to be just as demented as Aguirre himself.
- Combined with Ironic Echo in Avengers: Age of Ultron. Having already used "I've Got No Strings" to illustrate his newfound independence, Ultron can later be heard singing a creepy reprise of the song during the final battle, undergoing a Villainous Breakdown while strafing the Avengers.
- By day four of The Blair Witch Project, the stress of being lost in the woods with no map and being attacked at night by something has pushed Mike and Josh to the brink of a meltdown; as such, Heather's insistence that it's impossible to stay lost in America prompts Mike start loudly singing "The Star-Spangled Banner." The two men are still singing a cut later.
- Another famous instance of this trope appears in A Clockwork Orange, in which the undeniably psychopathic Alex DeLarge bursts into a house, assaults the homeowner at length, smashes up a good deal of his personal belongings, and then forces him to look on as he rapes his wife - all while merrily warbling "Singin' In The Rain."
- The second half of The Fly (1986) begins with Seth Brundle well into his fateful mutation and most definitely at the end of his psychological tether; when the time comes for him to reveal the source of his problem to Veronica, he absently begins singing "I know an old lady who swallowed a fly/perhaps she'll die..."
- Towards the end of Highlander, the Kurgan captures Brenda and goes on a joyride through the city streets, playing chicken with traffic and running cyclists off the road, rounding off the whole experience by singing "New York, New York."
- In In The Tall Grass, Ross Humboldt is eventually revealed to have been driven completely insane by touching the rock at the center of the field, and when the others refuse to join him in worshiping the rock, he begins stalking them across the field - merrily singing "Midnight Special" as he catches up.
- One of the many ways Jack Griffin demonstrates his monocaine-induced insanity in The Invisible Man is by singing while causing chaos: in one instance, he chases a terrified woman down a country lane while dressed in nothing but a stolen pair of trousers, singing "Here We Go Gathering Nuts In May" as he skips after her; in another, he steals a large amount of cash from a bank and nearly sparks a riot by throwing it at passers-by, gleefully belting out "Pop Goes The Weasel" as the crowd scrambles for the money.
- Already quirky and eccentric, Salvatore of The Name of the Rose is left completely broken after his torture at the hands of Bernado Gui; when the time comes for him to be executed as a heretic alongside Remigio and the nameless girl, he's so disengaged from reality that he can only sing a lullaby as the pyres are lit.
- Not long after being exposed to Satan's liquid form in Prince of Darkness, Calder begins loudly singing Amazing Grace in a desperate attempt to hang onto his sanity. Creepily enough, he can't finish the verse of "I was lost but now I'm found," the last word dissolving into demented giggling. Then he stabs himself in the throat, and rises again as one of Satan's mindless zombies.
- Reservoir Dogs features a rather infamous scene in which the demonstrably psychopathic Mr Blonde tortures the captive police officer and slices off his ear while singing along to "Stuck In The Middle With You."
- For most of Sherlock Holmes: A Game of Shadows, Professor Moriarty has been very careful to hide his "moral insanity" beneath a veneer of good manners; however, after catching Holmes infiltrating his base of operations, Moriarty finally lets the mask drop, merrily singing along with Schubert's "Die Forelle" as he tortures his nemesis with a meat hook.
- The introduction of the Beast in The Magicians firmly establishes him as a serious threat to the students of Brakebills when he magically freezes time in Professor March's class, leaving everyone inside fully conscious but unable to move, and devours one of the students alive for good measure. However, he's also revealed to be a pretty obvious example of a Psychopathic Manchild during his exit, in which he can be heard cheerily singing the the nursery rhyme "Bye, Baby Bunting."
- Stephen King has a flair for this trope:
- In The Green Mile, William "Wild Bill" Wharton can be heard singing mocking songs about the horrific death of Eduard Delacroix - scant hours after the body has been removed from the chair.
- During Henry Bowers' stay at Juniper Hill in IT, he notes that the pyromaniac Benny Beaulieu has gotten into the habit of half-screaming half-singing "try to set the night on fire", all while masturbating until he bleeds.
- Having been jailed just prior to the apocalypse in The Stand, Lloyd Henreid ends up trapped when the virus wipes out everyone else at the prison, eventually finding himself trying to pick meat off the arm of the prisoner in the neighboring cell, all while continuously singing the nonsense chorus lyrics from "Camptown Races."
- In the Lord Peter Wimsey novel "The Unpleasantness at the Bellona Club", one of George Fentiman's PTSD episodes found him dancing naked in a field and singing to the sheep "like a hoarse and rumbling wind in a chimney".
- Boardwalk Empire gives Gyp Rosetti a spectacular Villainous Breakdown in "Margate Sands," during which he claims that he's actually "Barney Google With The Goo-Goo-Googly Eyes." Then, after calming down, he begins singing the Barney Google song in an increasingly demented fashion, finally screaming the refrain. And then one of his three remaining henchmen stabs him in the back.
- An early scene in the Doctor Who serial "Ghost Light" features the character of Gwendoline singing "That's The Way To The Zoo" and accompanying herself on the piano. The fact that she's doing this with no audience and no logical prompting serves as a clear hint that she's not as stable as she looks. Of course, given the eventual fate of Reverend Matthews at the hands of Gwendoline and Josiah, this probably also counts as foreshadowing.
- Stark introduces himself in "Nerve" by repeatedly explaining the nature of his side of the room, then scurrying off into a corner and singing gibberish in soprano. Later subverted when it turns out that Stark intentionally plays up his instability so that the guards won't bother him.
- John Crichton has a habit of singing when his sanity drops a notch. For example, in "Crackers Don't Matter," when the entire crew of Moya begins cracking up as a result of the Monster of the Week, Crichton gets angry about Rygel obsessing over the crackers and starts yowling "I Scream, You Scream, We All Scream for Ice Cream."
- After being trapped on an isolated world with Aeryn for decades on end in "The Locket," an elderly Crichton exasperatedly sings "The Maid Of Amsterdam" as he waits to be rescued for the umpteeth time.
- A rare instance of this actually being a good sign occurs in "A Clockwork Nebari." After being mind-cleansed by the Nebari, Crichton is able to resist the drug's influence and - after a bit of Obfuscating Stupidity - reintroduces himself to Chiana by doing a little disco dance and singing "She's gonna gitcha-gitcha-gitcha-gitcha!" proving that he's every bit as crazy as he used to be.
- The final scene of "Liars, Guns And Money 3: Plan B" features Crichton suffering a meltdown as the Scorpius neural clone begins taking over his mind, trying vainly to shut out the voices in his head by singing "Daisy Bell" and growing more and more upset as his attempts fail - until he finally begs D'Argo to kill him.
- Season four of Grimm introduces the demonic spirit of Jack the Ripper, who turns out to be every bit as violent and psychopathic as his reputation suggests; after being discovered to be possessing Captain Renard, he kidnaps Sgt Wu and goes on a joyride through the streets of Portland, triumphantly chanting a Murder Ballad as he does so.
As I set out from London town/Upon a foggy day/I came across a fair young maid/And stole her heart away/But not the way you think it done/I did not use me charm/Instead I used a blade so sharp/She could not raise alarm! ALTOGETHER NOW!
- Mr Hyde is established as a fan of children's music early in Jekyll; not only does he introduce himself by whistling "Boys And Girls Come Out to Play," but he later kills a lion at the zoo, throws its body over the wall and onto a parked van outside, then spends the next few minutes clowning around in the animal's blood whilst gleefully belting out "The Lion Sleeps Tonight."
- During a visit to David's mind in Legion "Chapter 5", Syd stumbles upon David's image, singing "The Rainbow Connection" and accompanying himself on the banjo. From his tone of voice, it's clear that he's terrified beyond description of something Syd can't see, but it's not until later that we realize why: the Devil with the Yellow Eyes is taking over, once again driving David to madness.
- In the Red Dwarf episode "The Last Day," the replacement android Hudzen-10 has worn out his sanity chip after thousands of years spent trying to catch up with the ship, and when the crew refuse to allow him to replace Kryten, he attacks head-on while crooning a lullaby.
- In the Starsky & Hutch episode "Lady Blue," a woman's mind is destroyed by a "narco-hallucinogenic highball" and she's reduced to singing "I know where my head is at, won't somebody find my body?" over and over again.
- A more comedic example occurs in the Star Trek: The Original Series episode "The Naked Time". Kevin Riley, having been affected by the problem of the week, holes up in engineering, pronouncing himself the captain. As further evidence of his loopiness, he drives the crew nuts with his continual and badly sung renditions of "I'll Take You Home Again, Kathleen."
- One anecdote about the Nephandi in Mage: The Ascension features an Iteration X operative being witness to a virtual Eldritch Abomination devouring a swathe of the Digital Web... all while an innocuous-looking man stood in the background, smiling to himself and humming a children's song. As other books make apparent, Nephandi can range from chaotic Psychopathic Manchild-types to ice-cold psychopaths, and while none of them are as delusional as Marauders, they're much more dangerous.
- In Vampire: The Masquerade, the Malkavian clanbook often features segments written by the Lunatics themselves, including their views on other clans; when the time comes to describe Malkavian views on the Toreador clan, however, the narrator abruptly bursts into song - specifically to the tune of the Toreador March from Carmen!
Toreador, go bathroom on the floor-o! Toreador! Toreador! Sorry, just had to get that out of my system...
- World Of Darkness: Innocents
- The main villain of the sample scenario "With A Song In My Heart" is John Clerk, an immortal serial killer driven by the need to amplify the music he hears in his head via murder. As such, singing is one of his Character Tics, but since he's the player characters' mild-mannered music teacher, nobody finds anything particularly unusual about this habit... right up until the characters find themselves stranded in the middle of nowhere, at night, with the corpse of Mr Clerk's latest victim in front of them, and that song slowly closing in...
- From the same book, the eponymous villain in the scenario "Mountain Mother" is also prone to this: an ancient subterranean being believed to be the last of her kind, she's taken to kidnapping children in an effort to raise them as her own, and her most recognizable lines are described as "eerie singing in a language that hasn't been spoken in tens of thousands of years" - an indication of just how lonely she's become over the eons.
- In one of the oldest examples in fiction, Hamlet features Ophelia being driven insane following the collapse of her relationship with the title character and the murder of her father; true to form, her very first lines in Act IV are the lyrics of a love song. In fact, almost all of her lines from hereon until her suicide consist of insane singing.
- Batman: Arkham Series:
- Playing Batman: Arkham City on New Game Plus all the way to the end eventually results in a stinger in which Harley Quinn, heartbroken over the death of the Joker, can be heard singing "Hush little baby, don't say a word/ Mama's gonna kill for you the whole damn world."
- In Batman: Arkham Knight the hopelessly deranged Professor Pyg is introduced singing tunelessly along with his favorite opera music while performing nightmarish plastic surgery on one of his victims. Also, after being arrested and deposited in the GPD lockup, Pyg can occasionally be heard repeating the song - much to the annoyance of the other captured supervillains.
- The Splicers of Bioshock have a habit of singing to themselves, each model having a different song:
- Toasty, obsessive romantic that he is, will occasionally croon "You're The Top" and My Dreams Are Getting Better All the Time" alongside meaningless nonsense tunes.
- Baby Jane can be heard sing-songing "a-five six, seven eight, a-five six, seven eight, seven eight..."
- Dr Grossman's Bioshock 2 incarnation will sometimes sing about "rusty tumors hiding from the Lamb."
- When she isn't grumbling about servants not knowing their place, Lady Smith can be heard muttering the lyrics to "It's Bad For Me."
- Rosebud sings different sections of "Amazing Grace."
- As a religious fanatic, Waders can often be heard croaking out the first stanza of the hymn "Jesus Loves Me."
- Fancying himself the CEO of Fontaine Futuristics, the extravagantly deranged Dr Gilbert Alexander mockingly shrieks the the company advertising jingle when you try to break into the building, at once preventing you from using the recorded password and shamelessly showboating.
When you're down in the mouth and life's a pain/Weatherman says heavy rain/A little boost is all you need/Average Joe to Hercules/Stronger arm and sharper brain/That's why the future is FONTAAAAAAAAAAAAINE HAHAHA!
- Eddie Gluskin of Outlast: Whistleblower makes use of this as one of his most terrifying habits: after you manage to escape his initial patrol, he begins singing a haunting rendition of "I Want A Girl" as he stalks you across the vocational block. Quite apart from the fact that he's audibly expressing his obsessive lust for you, it's really difficult to tell just how close or far away he is, because his singing seems to come from everywhere at once...
- Sigma in Overwatch is a brilliant astrophysicist, but one of his experiments briefly created a black hole, and that very brief exposure left him with severe psychological damage. This leaves him often humming some strange tune that only he can hear, which he claims is the mathematical music of the universe itself.
- Both the trailer and the main body of The Park feature Lorraine Maillard nervously reciting the final verse of "Five Little Ducks" as she progresses through Atlantic Island Park, all while suffering a very nasty case of Sanity Slippage courtesy of Nathaniel Winter mind-raping her over the course of the story. For good measure, her cameo appearance in The Secret World makes it clear that the song is still very much embedded in her mind: her suicide note refers to her long-dead son as "little duck".
- In Spec Ops: The Line, the airwaves of Dubai are currently under the control of the Radioman, who appears to have gone a little crazier than most of the city. Along with singing his own jingle to introduce himself, during the helicopter attack on Captain Walker's team, the Radioman plays "Dies Irae" over their earpieces... and proceeds to sing along - badly.
- Pokecapn's famous Sonic the Hedgehog (2006) Lets Play exhibits pronounced Sanity Slippage as the bugs, badly-implemented controls and difficulty spikes slowly take their toll, eventually forcing some of the players to resort to song as a coping mechanism: pokecapn himself takes to blaring out an a cappella rendition of Flash Man's theme whenever boring gameplay begins to get on his last nerve, while KFJ's journey through the End Of The World level is punctuated by him reciting the lyrics to various songs in a dead monotone, concluding with "Land Down Under." And after finally winning this section, the entire group bursts into song.
- In Justice League, John Dee's failed attempt to wipe out the Justice League as Dr. Destiny leaves his mind broken and himself imprisoned in a catatonic state, endlessly humming "Frere Jacques" over and over again.
- In American Dad!, "The One That Got Away", Sidney Huffman has his life ruined by Roger (unaware that Sidney's a Split Personality of his), and he's soon reduced to sobbing on the floor while singing "The Lord is Good To Me".
- The Gravity Falls series finale features Bill Cipher belting out a refrain of "We'll Meet Again" while reintroducing himself to Stanford Pines. Definitely an example of the third kind, for while Bill is already a megalomaniacal psychopath, his impatience to get out of Gravity Falls has made him even crazier than usual.
- Recess: In "The Box", T.J. is forced to spend time in a "box" drawn on the blacktop. By the end of his sentence, he's in a Troubled Fetal Position while singing "This Old Man".
- Todd does this in Wayside while suffering Sanity Slippage from being the kindergartners' king; by the time Myron finds him, he's singing "Patty Cake" in a Troubled Fetal Position.
- It's not unknown for schizophrenics to sing or hum insistently; however, though this behavior appears irrational to observers, it's actually a subversion: people who suffer from auditory hallucinations need a method of drowning out the stimuli until it passes, including talking, humming, or singing. In some cases, it's actually considered a legitimate coping mechanism.