An easy way to demonstrate insanity in a character is using an inconsistent speech pattern, or even multiple speech patterns that the character rapidly switches between. The character's erratic behaviour regarding their speech and mannerisms may also disturb other characters, especially when switching mid-speech. It's also used as an indicator of Split Personality, or Many Spirits Inside of One.
Rapid mood swings are also a characteristic used this way, and frequently associated with temporary stress-induced insanity. Suddenly SHOUTING! is a common overlap, when a character switches from calm to angry, and back again.
Not to be confused with Voice of the Legion where there is in fact more than one speaker co-habiting the body or Voices Are Mental, where the minds of the persons have switched over. Also does not overlap with Flip Personality, where the rapid changes are used to indicate that the character is some form of possessed, not insane. Supertrope to Talking to Themself, where the character is knowingly holding a conversation, and switching personalities in order to do so.
- In Savage Dragon, the title character's Distaff Counterpart, She-Dragon, would to all appearances be talking to herself in different voices, all of whom had their own distinct word balloons and personalities. She was believed to be crazy for years, but it turned out she was speaking to inter-dimensional beings.
- The Hellion of The Land of What Might-Have-Been speaks in a wildly-distorted tone of voice, described as deep and thunderous one moment then high and sing-song the next. For good measure, her dialog is conveyed in a patchwork of italic, bold or underlined text, just so you recognize the Hellion's voice long before she appears.
- The Mad Hatter in Tim Burton's Alice in Wonderland (2010) occasionally breaks into a Scottish brogue, usually to deliver defamations or invective.
- Toecutter in Mad Max has a different accent in each scene. Actor Hugh Keays-Byrne decided to Throw It In! to demonstrate his character's insanity.
- In the film adaptation of the Israeli novel Phobydillia, after Grumps reveals the landlord's secret, Regev remarks that it's one hell of a twist and he soon starts to blurt out a string of pop-culture quotes, changing inflections to mimic the original delivery, in a state of anger and emotional instability.
- Komodo, the insane, self-obsessed warlord of Warriors of Virtue, uses many different voices throughout the film.
- At the climactic Elstyn family meeting in Aunt Dimity Takes a Holiday, only Simon, the recipient of the threatening notes, takes a cup of tea offered by the maid. That maid is revealed to be Derek's insane former nanny, who targeted Simon in the belief that he was trying to take Derek's inheritance. She rambles insanely about Simon: "I tried to warn him, but he wouldn't listen. Won't listen must be made to listen." And follows that with a Stage Whisper order to Derek: "Make him drink his tea...." Once she leaves the room, an Inspector from Scotland Yard is disturbed enough by the behaviour that he asks everyone to avoid touching the teacup, since the police intend to have it analysed. Afterward, there is some speculation among the other characters about whether she will be legally judged insane.
- Children of Dune: When Leto confronts Alia, she starts making disjointed statements and speaking in different languages as the multiple personalities inside her all try to talk at once.
- Discworld's Canting Crew features Altogether Andrews, a beggar and burned out psychic who was overrun with spirits. Andrews' facial expression and style of speech changes as his body passes between the controls of the various personalities.
- Good Omens features Witchfinder Shadwell, whose accent is described as a random, shifting mixture of accents from all over Britain. His deputy, Newt, finds him unlikeable and possibly unstable, but considers operating as a "witchfinder" to be decent enough pay to tolerate a boss who believes witches are real and need to be burned.
- The Sherlock Holmes short story, "The Adventure of the Dying Detective", features Sherlock giving instructions to Watson on what man he is supposed to fetch, interspersed with calm musings on oysters covering the world or excitedly telling him to shift the coins in his pocket to be better balanced. He's faking delirium to convince Watson that he's near death, to trick the murderer into confessing to him.
- Padan Fain, aka Mordeth, Ordeith or Shaisam, is the Big Bad Wannabe of The Wheel of Time. As a result of a very unusual sort of Fusion Dance, he's a composite of two very broken souls, and as such changes accents frequently, usually with his mood and usually without his noticing.
- The Monster of the Week in Doctor Who episode "The Face of Evil" is Xoanon, an intelligent computer who was in the process of developing a personality. When the Doctor tried to fix it by connecting it to his own brain, it broke instead, giving it a copy of his own personality which conflicted with its own newborn intelligence. As a result, when he returns during the events of the story, the computer has multiple conflicting personalities and is batshit insane. To indicate this, the computer has multiple voice actors, including the Doctor's actor himself, who randomly switch out midsentence while the computer is speaking.
- In the Kung Fu: The Legend Continues episode "The Secret Place", the villain is a crazy man who has taken a busload of students hostage. His insanity manifests by changing his voice/personality while quoting from movies & tv shows.
- In Night Court Dan hooks up with a woman with multiple personality disorder, all of which are movie villains. Her voice changes as she goes from character to character.
- Moriarty from the BBC series Sherlock is generally depicted as Ax-Crazy. One of his more peculiar habits is to sometimes change accents, from an Irish accent to an RP London accent to, most strangely, an American accent (he does this during his Rooftop Confrontation with Holmes in "The Reichenbach Fall").
- Baldur's Gate. Xzar of Zhentarim is utterly insane, and delivers most of his hammy lines in different voices and tones to indicate his insanity, even quoting Hannibal Lecter.
- In Carmen Sandiego: Great Chase Through Time, Jacqueline Hyde shows her multiple personalities while being jailed. Her gentle apology abruptly switches to a rough, mocking, Evil Gloating.
- At the end of Danganronpa: Trigger Happy Havoc, the true Mastermind behind everything switches rapidly between personalities, including a cutesy teenage girl, a tomboyish punk, and a haughty condescending personality, each with its own vocal patterns and tics. In the dub, she has two voice actresses to accommodate this and the sequel adds a third.
- In Fallout, the Master suffered from an extreme exposure to the FEV virus, and ended up being reduced to just a sentient blob of flesh that somehow managed to fuse himself with the machinery in his hideout. When you finally meet him, his speech is constantly alternating between a normal male voice, a male computerized voice, and a female computerized voice.
- In the Outlast: Whistleblower Expansion Pack, at one point Waylon is sneaking around and hears several inmates plotting to turn over anyone they find to Eddie Gluskin in the hopes that he'll leave them alone. Eventually we find out it's actually just one guy talking to himself in multiple different voices/accents.
- The Portal character GLaDOS is slowly revealed to be insane to the player, and 'her' rapidly shifting tones and moods foreshadow this.
- In Rune, Loki gives his Evil Gloating of his master plan in this manner underneath the bowels of the Earth, going from sing-song voice to violent outbursts and back again in a manner of seconds, having been driven insane by his captivity by Odin.
- Gamzee Makara from Homestuck, when sane, speaks WiTh AlTeRnAtInG cApItAlIzAtIoN, but when insane, speaks JUST like THIS. One of his victims explicitly comments on the change in his intonation; and a single word all in lowercase is enough to make Karkat start panicking.
- Questionable Content's Randy normally speaks in an excited tone of voice, but during one exchange with Yelling Bird, his text size drops dramatically, speaking in all lowercase (except for the word "I"). The claim he made during this exchange was that he is eternal and he might not be wrong.
- The Care Bears: Adventure in Wonderland. The Care Bears meet the Mad Hatter in Wonderland, who performs a song assuming many mannerisms of different personas brought by simply changing hats, to accentuate his particular insane fixation on hats.
- HIM from The Powerpuff Girls switches from sugar-sweet effeminate giggling to a very deep, menacing voice in very sudden and unpredictable ways.
- Batty Koda in FernGully: The Last Rainforest changes accents/opinions in mid-sentance as a side-effect of the electric Animal Testing he used to endure. But the Doylist explanation is that he was voiced by Robin Williams, and the company gave him an insane character so he could use his many voices in one role.