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"He kept giving these line readings with all this inflection in them. I kept telling them that it had to be less, a lot less—like a robot. He kept saying it sounded so flat. Everybody else was looking at me too, and was asking me if I was sure. To them it sounded flat. I think it really sells it."
Normally, people who are calm tend to speak in a clear, even voice that doesn't vary too much from word to word and is immensely reassuring to the listener. This isn't always a good thing, however. A perfectly monotone voice usually means just the opposite of sanity, safety, and cause for reassurance; usually, it means the speaker is mind-controlled, crazy, an impostor, or is being threatened with death. In these cases, the creepy monotone is a sort of Glamour Failure to clue in the hero that any of a dozen things may have just gone wrong.
Usually, the creepy monotone varies from a calm monotone in that no word or syllable is stressed at all, even when they should be. This is the difference between saying "Oh hello!" and "Oh, hello." Even Spock Speak sounds more natural and less unsettling by comparison. Contractions are often omitted, as are specific or explicit mentions of names and facts, depending on the cause of the monotone. Here are some common scenarios for the Creepy Monotone.
"I am sorry. If it's any consolation, this likely won't hurt much." If a character is introduced with a creepy monotone, then all bets are off. If you're lucky, they're just an Emotionless Girl or The Stoic, but odds are they're an Evil A.I., Ax-Crazy, or some other seriously dangerous character.
Digimon Frontier's Dark Trailmon. He's actually very HAL-inspired, in design and in voice. (Ironically, he's not evil, just creepy: you expect the Dark Trailmon that you meet in the Dark Terminal during a hallucination caused by Duskmon, the Warrior of Darkness, after he ambushed you and kicked your butt on the Dark Continent to be a little... dark.)
Prior to that, Adventure had Machinedramon, who was dark.
Many Nightmares from Digimon Tamers came from J-Reaper, who was even darker.
Legato Bluesummers of Trigun almost always spoke with a monotone (or simply very calm) voice.
Paul von Oberstein from Legend of Galactic Heroes has a trademark Creepy Monotone and is one of the grey characters of the series. While he has thrown his lot in with Reinhard, he also has his own goals and apparently has little need for the fellowship of the other admirals. His actions lead to the death of Siegfried Kircheis, Reinhard's closest friend in an assassination targeting Reinhard. He is also a participant to the events that lead to Oscar von Reuental's rebellion, although he later exhibits something not unlike sympathy.
In Higurashi no Naku Koro ni, speaking in Creepy Monotone tends to show up in the main cast whenever things about the Town with a Dark Secret or Oyashiro-sama's curse start getting unearthed. More specifically, those under the influence of Hinamizawa Syndrome start imagining the people around them doing this, compounding their paranoia.
The Anti-Spiral leader in Tengen Toppa Gurren Lagann, and Nia when she's changed into an Anti-Spiral. Once the protagonists get the better of him, he snaps and starts getting Hot-Blooded.
R. Dorothy Wayneright from The Big O is monotone because she's an android (well, gynoid, technically). However, she is notemotionless, making it hilarious when she verbally abuses Roger in a calm, soothing voice.
Both of Aiko's parents in Vampire Princess Miyu speak in very flat, unemotional voices and have very slow speech patters; this makes them sound horribly creepy, even when they seem to be very kind persons aside of this "detail". This very subtly hints that they are actually Dead All Along.
Vamp! has the character of Shizune, who is notorious for speaking with one of these.
Rorschach from Watchmen speaks in a low, growly monotone (other characters remark on its unsettling quality) with odd, near-Hulkspeakclipped sentences. "Yes. Good night. Remember it often." He has flashes of eloquence, too, which are unnervingly at odds with this style of delivery.
His mode of speaking is shown by his unique speech bubbles, with no words ever bolded or italicised for emphasis like the other characters'. By contrast, his speech bubbles in flashbacks to before the Blaire Roche case indicate a totally normal voice.
In Men In Black, the bug in the Edgar suit demands sugar water from Edgar's wife. OK, so his creepy monotone is very gruff and hostile-sounding, but that basically matches Edgar's pre-bug personality as well.
Any and all characters in Sin City, including the heroes, have an extremely dry and calm manner of speaking, bordering on creepy monotone. This is exemplified with Marv, who always speaks in a soft, calm manner, bordering on the nonchalant, even when brutally torturing and murdering somebody. Roark Jr, before his run-in with Hartigan, speaks almost like HAL. After that, though, he's a lot more expressive.
HAL 9000 of 2001: A Space Odyssey speaks in an eerily calm and soothing voice, though he occasionally displays some emotions: prideful and even apologetic and, at the last, fearful, making him the most developed character in the film.
Jonathan Crane in Batman Begins is this after he's been given a massive dose of his own fear toxin and Batman, who's already kind of intimidating, is interrogating him.
After Selina Kyle is almost killed by her boss Max Shreck in Batman Returns for knowing too much, she repeats the same words that she said upon entering her apartment in a previous scene, only in a creepy monotone of the first variety: "Honey, I'm home. Oh, I forgot. I'm not married." This turns out to be a prelude to Selina's impending Freak Out and transformation into Catwoman later in the scene.
Bart, a rogue cop in Southland Tales flatly insists on tagging along to a (fake) domestic disturbance with Taverner and Santaros and executes the couple on arrival. A creepy monotone delivered by Jon Lovitz is surprisingly unnerving.
The evil townspeople of "Nilbog" also do this in Troll 2 while trying to force the protagonist to eat a poisoned bowl of ice cream and, in the process, prove that even a creepy synchronized monotone can turn into hilarious narm (or maybe even disturbing Fetish Fuel) with the right line:
"Open your mouth, my little friend! Please, open it!"
Andy gives us this lovely line in Toy Story 2: "I don't want to play with you anymore."
The robot GERTY in Moon is voiced by a very monotone-sounding Kevin Spacey. In the end, the trope is subverted as GERTY turns out to be mostly benevolent, going so far as to perform a Heroic Sacrifice.
"Max", the AI of the spaceship from Flight of the Navigator, speaks in a dull, robotic monotone at first. Then, after doing a scan of David's brain, abandons this trope when he accidentally picks up some human personality traits and starts sounding like a robotic Pee-Wee Herman (naturally, as Max is voiced by Paul Reubens).
Sy Parrish in One Hour Photo spends the majority of the film talking in a very soft-spoken, almost robotic monotone, reflecting his disturbing lack of a personality, although he does show emotion at times, especially near the end of the film when he starts to lose it.
The aliens in the first Invasion of the Body Snatchers, when no real humans were around to fool. Such as "I put the pod in the baby's crib. Soon there will be no more crying."
The nameless narrator in Fight Club sometimes slips into this, both in his narration and his dialogue in the film.
Interestingly, Darth Vader talks like this in the first Hungarian dubbing of The Empire Strikes Back, even keeping up the emotionless tone during his dramatic confrontation with Luke. Nostalgic fans often cite this as their favorite Vader voice, but others claim it came off more as "reading a script" than "acting".
J. Robert Oppenheimer's "Now I am become Death, the destroyer of worlds" dialogue concerning the creation of the atom bomb comes off as this partly because he was saying it for the non-dramatic documentary The Decision to Drop the Bomb and partly because of the grim subject matter.
The titular protagonist in Lucy,after gaining superhuman mental abilities, speaks in a matter-of-fact monotone, whether discussing human biology with Morgan Freeman or torturing a drug kingpin.
Mr. Jonathan Teatime from Discworld unnerves people with his creepy monotone.
Lord Voldemort speaks in a calm, flat tone of voice; this, perhaps even more than his use of Soul Jars and his utter lack of compassion, makes him terrifying. Except for when he gets agitated. It comes across more as him trying to achieve this trope but failing every now and again.
As Crouch Jr. proved, people who have drunk Veritaserum (Truth Serum) also speak like this.
The demon from the Neil Gaiman short story "Other People" speaks in "a voice that carried with it no sorrow, no relish, only a dreadful flat resignation." It is utterly expressionless for all of the story.
Myrddraal from The Wheel of Time have voices that are described as sounding like crumbling bone and are utterly devoid of emotion or inflection. Not only is this incredibly creepy, but it makes Shaidar Haran, the only Myrddraal who does show emotion (because he's possessed by a "shadow", as Word of God describes it, of the Dark One), even more disturbing by comparison.
In the last book, the Sharan accent is described in this vein; a Sharan Ayyad who is part of Demandred's army is described as sounding like "all the music was gone from her voice."
Brinkley was at the door, begging me to come out and let him ascertain the colour of my insides; and by Jove, what seemed to me to add the final touch to the whole unpleasantness was that he spoke in the same respectful voice he always used. Kept calling me 'Sir,' too, which struck me as dashed silly. I mean, if you're asking a fellow to come out of a room so that you can dismember him with a carving knife, it's absurd to tack a 'Sir' on to every sentence. The two things don't go together.
In Murderess, James, the boy from Chopped Tree Inn, talks like this when welcoming Lu. His mother quickly sends him away when he scares Lu too much.
In A Song of Ice and Fire]], Lord Roose Bolton, being a devoutly stoicSoft-Spoken Sadist, always speaks in whispers so faint that people have to lean close to hear him. His tone is constantly described as unsettling, emotionless and cold, as he believes that avoiding passion is the key to staying young.
What makes it even creepier is that the monotonous "Bored now" is the exact line used by an alternate version of Willow wherein she was an evil badass vampire chick. Dark Willow was basically her vampire double sans the vampire.
Subverted. Half the fun of most dialogues the title character takes part in resides in the way Michael C. Hall delivers his lines, in an emotionless creepy monotone that should trigger the "serial killer alarm" in the minds of even the most oblivious of the other characters. Except, of course, it never does. "Why, in a building full of cops - all supposedly with a keen insight into the human soul - is Doakes the only one that gets the creeps from me?"
Inverted when he snaps and kills the used car salesman for insulting Rita. Cue scared "he's freaking crazy!" reaction.
Eugene Victor Tooms bare-ly has an-y in-to-na-tion at all and talks real-ly slow-ly. It's ser-i-ous-ly freak-y.
Donnie Phaster, one the show's creepiest villain. Go watch "Irresistible" at 2 AM if you have any doubts. His voice is calm and super creepy.
In the Firefly episode "Objects in Space", River announces to the bounty hunter Jubal Early that she has "melted away" into the ship Serenity, becoming part of it, in a creepy monotone. It becomes even more chilling when she alters her tone just a tiny bit for the line
"Not talking to River. You're talking to Serenity. And Early? Serenity is very unhappy."
Summer Glau is good at creepy in general. In The Sarah Connor Chronicles, Cameron regularly speaks in a constant, soft-yet-creepy monotone. However, the show also plays its absence to excellent effect. The absence of the creepy monotone we're used to is what made the scene where Cameron recites not only word for word, but inflection for inflection a "classmate's" emotional bathroom rant for the principal creepy.
Similarly, in the second season premiere, the steady shift from Cameron's creepy monotone to her desperate, terrified pleas to John to a screaming declaration that she loves him while he's trying to take out her chip makes for a chillingly awesome scene.
In fact, whenever she's not speaking in her monotone voice tends to be even more unsettling, because it's so obvious to the audience that she's completely faking it.
And in the episode "Allison From Palmdale", the use of the creepy monotone and its absence makes an already chilling episode that much creepier.
Deadliest Warrior gives us Brahm Galligher, who seems to be incapable of speaking in anything but a creepy monotone. Unless he's swinging a halberd around.
The Cybermen from "The Moonbase" onwards, and current. (In "The Tenth Planet" they spoke in a creepy emotionless sing-song modelled on early 'singing computers' like IBM 7094).
The Daleks use a mechanical, staccato voice delivery, but they tend to increase both speed and volume for emphasis. Also, there's quite a bit of variation of individual voices. Creepy, yes; intensely emotional, and very monotone - only the single tone used isn't flat apathy, but rather a consistent bent of shrieking hysterical rage. When they're not grating out staccato bursts of antipathy, it usually means something's very, very wrong.
The sociopathic and terrifying Himmler-Expy Nyder in "Genesis of the Daleks" has this as his affect. He's remembered as one of the scariest supporting character in the story because of this, even next to Davros and the Daleks.
The robots in "The Robots of Death" - one of the many reasons they invoke the Uncanny ValleyIn-Universe. The sequel Corpse Marker establishes that this monotone has become fashionable for humans to affect, especially those of upper classes.
The creepiness in "The Impossible Planet" consists mostly of the creepy monotone of the Ood and the computer system being possessed by Satan.
Used to great effect in "Silence in the Library", where an AI retelling cries of panic and pain in utterly emotionless creepy monotone made the situation seem scarier than it would be if they were reproduced verbatim.
Sylar from Heroes could possibly pass for a perfectly ordinary, non Serial Killer type, except that he seems capable only of a creepy monotone or murderous rage. No middle ground.
Sylar: This is usually the part where people start screaming.
The Observer in Fringe. On first seeing him, this trope leads the viewer to believe that he's got something devious going on, but he turns out to be more ambiguous than that.
Award for creepiest borg monotone goes to Locutus' line: "Your resistance is hopeless... Number One."
Almost everything Locutus said was terrifying.
It's worth noting that Picard used the Locutus monotone while interrogating the liberated drone Hugh, who recognized Picard as Locutus.
Lt. Commander Data also qualifies for this trope.
LOST gives us Benjamin Linus. "Of course, if I was one of them, these people that you just assume are your enemies, what would I do? There would be no balloon so I would draw them out to a really secluded place, like a cave or some underbrush, a good place for a trap... an ambush. And when your friends got there a bunch of my people would be waiting for them. Then they would use them to trade for me. I guess it's a good thing I'm not one of them, huh? ...You guys got any milk?"
Robert Stack would often host his segments on Unsolved Mysteries in what was supposed to probably be a very flat, newscaster-type voice. However, the tone of his voice and the creepy nature of the show sent it careening into this pretty quickly.
Tyler the Creator does this in his song "Fish," which details the thoughts of a man who is either a cannibalistic, necrophiliac serial killer and rapist, or who has an incredibly unhealthy obsession with fish.
This is one of the suggested options for the voice of Friend Computer in Paranoia.
On the few occasions in Warhammer 40,000 that Necron Lords have spoken, they have been unusually polite and cultured speakers possessed of a truly chilling calm, which may make some sense, given that they are the still-sentient leaders of a race that now exists as, for all intents and purposes, undead robots.
"Lucky creatures... At long last... you have found... the tranquillity... of death..."
In Phoenix Wright: Ace Attorney - Trials and Tribulations, Viola Cadaverini is said to speak with a creepy monotone, laced with subtle death threats.
The G-Man from the Half-Life series is a variant of this, particularly in Half-Life 2. He speaks in semi-monotone but stresses the wrong words and pauses (and fails to pause) in all the wrong places. (ex. "Not that I wish to-imply that you have been... sleep-ing on the-job")
A version of this appears in Half-Life. Black Mesa's PA system is created via a sort of voice synthesizer that assembles pre-existing voice files into phrases. This is initially used as a generic announcer, but it is later taken under control of the enemy soldiers and used to threaten the protagonist himself.
The Combine announcer combines this with medical terminology: She/it sounds like she's/it's describing a medical procedure, not an urban counterinsurgency crackdown.
Max Payne, in both games. It's particularly striking in the first game, since the actor providing his likeness, Sam Lake, looks quite humorous and has a flamboyant outfit (leopard prints).
This is how the protagonists know that something is wrong in the titular star system in the game Nova 9. They get a brief Distress Call that is cut off, then, shortly after, receive a message delivered in creepy monotone saying that everything is fine and to disregard the previous message.
The animated Statues in Thief: Deadly Shadows. They have one purpose only and, as a result, come across as rather single-minded. "A noise and find and crush and kill and crush and kill..."
Now, Sheogorath from Oblivion is known as the Lord of Madness, so unbalanced tendencies are to be expected. Normally, in fact, he's rather charming and downright giggle-worthy with his fake-Scottish accent and habit of cheerily threatening life and limb (and mental integrity). However, when he suddenly becomes very quiet, his eyes narrow, and he speaks very, very softly, you're reminded that you really ought to be terrified of this guy.
Portal proudly sports GLaDOS, who has a sort of musically random inflection, but in a very mechanical way. This is emphasized when she uses a word or phrase more than once in a short period of time. Also, "Wheeee..."
The turrets also have the same type of voice (and the same voice actress), but with a more cute than creepy inflection (which doesn't say much).
You spend so much of the game listening to that voice, right from the very start, that when you destroy GLaDOS' Morality Core and she switches from robotic monotone to an emotive, fluent female voice, it's actually far creepier than the monotone, because GLaDOS talking like a normal person is simply not right.
In BlazBlue, the "final boss" in Arcade Mode is ν-13 (pronounced "nu-thirteen"), who is introduced with a creepy monotone and, yes, is an Emotionless Girl with an Evil AI who becomes Ax-Crazy around certain people. Like GLaDOS, ν-13 shares a voice actor with another character.
Nu does drop the creepy monotone whenever Ragna the Bloodedge is around, and uses a bright and cheery, almost babylike, voice with him. This is not an improvement.
Lambda-11 of the sequel shares the same voice actress as Nu and speaks in a creepy monotone as well, albeit in a slightly different tone.
The Many speak this way in System Shock 2. They have multiple voices (an electronically-pitched rumble, a whispering masculine voice, and a nearly singsong feminine voice) and one or two of them sometimes have inflection, but the majority of it is in a serene monotone. "How can you choose cold steel over the splendor of flesh?" The contrast between the Many's monotone and SHODAN's stuttering and wildly varying inflection and pitch makes for one of the creepiest aural dichotomies the gaming world has to offer.
Also, the Many-controlled AI, Xerxes (who resembles HAL 9000 much more than SHODAN): "Decompression in five seconds. We regret any inconvenience." He is also quite dissonantly serene; he announces workplace accidents, the number of shopping days until Christmas, meal times, and security breaches in the same tone as the freaky messages programmed into him by the Many.
The original System Shock also had a generic voice announcer on Citadel Station that spoke in this manner, alongside SHODAN herself.
In the Kingdom Hearts series, nearly every single Organization XIII member talks like this at least once, with Saix being the most notable example.
Xemnas' voice never has much emotion behind his droning voice, thanks to Paul St. Peter's acting. It's almost more disturbing than Saïx.
"There's no such thing as light." (He says this while surrounding Sora and Riku in an Up to ElevenBeam Spam.)
Young Xehanort in Dream Drop Distance is a particularly jarring case, as all other versions of the character are infamous for being Large Hams. Even Xemnas somehow manages to ham it up while monotone, but not this guy.
The promo material for Mass Effect 2 features a geth named Legion speaking in this manner. It is absolutely terrifying when you hear him speaking on how the geth use fear in such a cold mechanical voice. Then you play the game and find out the promos lied to you, and Legion's voice just makes it adorable.
Legion: We do not experience fear, though we understand its effect on you.
The Reapers speak this way as well, particularly Sovereign. Harbinger carries slightly more inflection, but is still consistent in tone whether he is ASSUMING DIRECT CONTROL or remarking on how irritating the protagonist is.
"Shepard. You have become... an annoyance."
In Mass Effect 3, the Leviathan DLC has an entire mining facility speaking like this. This, along with the fact that they are unnaturally calm as Reapers attack their front door, are major hints that something is very wrong. It's also hinted that they've been speaking this way for ten years.
The Tranquil in Dragon Age: Origins - Mages who have undergone a sort of magical lobotomy and are now incapable of emotion. The Sloth Demon also has one.
One word: Hespith.
First Enchanter Irving has a bit of this, though it makes him sound more like a tired old man rather than crazy.
Algalon of World of Warcraft, up until he loses his cool and admits his convictions may have been in error. He'll unleash hell on you before that happens, though.
S.T.A.L.K.E.R: Call of Pripyat: You don't want to hang around The Monolith Fighters, but there are moments where their Creepy Monotone can be quite memorable.
The "Milkman Conspiracy" level from Psychonauts is the mind of a paranoiac, where Government agents with the flimsiest of disguises spout hilarious monotone oneliners to maintain their cover.
The Martians of Jeff Wayne's War Of The Worlds speak like this. A notable exception is their trademark "ULLAAA!" from the musical, which can sound triumphant or sad depending on the situation.
A minor example, being slightly unsettling instead of a major warning sign, but in I Miss the Sunrise, Mr. Right and Mr. Left seem to speak in a robotic monotone, indicated by the fact that their sentences appear instantly instead of letter by letter.
Sophie in Tales of Graces normally speaks in something of a monotone anyway, but goes to creepy when Pascal brings up the idea of Richard eating royal crablettes. "Royal crablettes... royal crablettes..."
Cortana from the eponymous level in Halo 3. Although most of her lines are anything but monotone she says, "This is UNSC AI Serial Number CTN 0452-9. I am a monument to all your sins." in a monotone with bonus points for also using Voice of the Legion vocalization.
Against Scarlet Queen in Segment 2 LEAD, if you unlock her Spirit Kernel form.
At the end of the Shrine of Farewell, if you've killed all four bosses within the time limit.
Against Rex Cavalier in Segment 7, as the final phase of his assault begins.
Homestuck's Gamzeepossibly speaks in monotone; it's hard to be certain exactly how his text quirk would be vocalised. At any rate, he later goes off the deep end, and upscales his wHiMsIcAl CaSe AlTeRnAtIoN to whole lines of text, meaning that he alternates between barely audible and screaming in rage.
Many people believe that Doc Scratch speaks like this. Well, when he's not flipping his shit.
0f all the pe0ple in h0mestuck t0 menti0n here gh0st aradia sh0uld be at the t0p 0f the list
Mr. Freeze on Batman: The Animated Series never really deviated from his cold, chilling (no pun intended) monotone, providing juxtaposition to the emotional depth of the character. It was effective. Word of God has it that his voice actor, Michael Ansara, struggled with this element of the role — his instinct was to act, and he had to be constantly reminded to be flat and robotic.
Possibly especially so in Batman Beyond, where he stops being simply apathetic and starts being straight up merciless.
In one of The Simpsons' Halloween specials, Bart is turned into a vampire. He rejoins the family and monotones, "Hello mother. Hello father. I hope you were well during my brief and uneventful absence."
Then there was the one where Lisa has to tell Homer that Bart had rented a car with a fake ID and was trapped in Tennessee. Homer's face goes red and he says calmly, "Yes. That's a real pickle. Would you excuse me?" He puts on a radiation helmet and screams incoherently for a while, causing the faceplate of the helmet to fog up. He then takes it off and says, now using the monotone again, "Alright, I have thought this through. I will send Bart the money to fly home. Then I will murder him."
The Twins from Superjail speak in a low, quiet, gentle monotone voice - a stark contrast to the Warden's loud, flamboyant, inflected way of speaking.
While everyone might want to forgetMega Man's "Curse of the Lion Men", it does fit here. After Tar flash-reprograms Wily's robots and Roll, Protoman very flatly tells him that he's now their leader. Made even more unsettling since no one else being mind-controlled during that episode invoked this trope (except perhaps Roll).
Earlier than that, Mega Man's voice becomes flat (along with a Brief Accent Imitation) when he was under Wily's control.
Also parodied in the Futurama episode "Raging Bender", when Hermes comes back from vacation with a green slug on his head and explains in a creepy monotone that they had to make an emergency stop on the Brain Slug Planet. Throughout the episode, he tries to trick the others into wearing brain slugs.
The episode "Operation: D.A.T.E" has the Delightful Children using a Mind-Control Device on a lot of kids they invited to a party in their mansion. As a result, they get Mind-Control Eyes and keep saying "Join us..." in a monotone sounding voice.
Ultron in The Avengers: Earth's Mightiest Heroes. Not helping is that he can't change his expressions, and his head is designed to look like an ant. Even worse, before he turns evil, he speaks in a much more emotional and upbeat fashion, meaning this is most likely intentional on his part.
Her flat-tone delivery is most likely one of the reasons why Daria is so unpopular in school. The audience usually finds it endearing, but to most other people, she's "the freaky quiet girl".
Yakko: To yield is to allow oncoming traffic the right of way.
Dot: Your breath is like the breeze of a landfill.
Yakko: Food particles are wedged between your teeth.
In Wakfu, Rushu is normally a hammy villain. When he is forced to revert to his true form while fighting Goultard, he briefly slips into a cold monotone, the implication being that he's not really having fun anymore.
Ignignokt, the green mooninite from Aqua Teen Hunger Force.
The Regis Mark V in Megas XLR speaks exclusively with a rapidly-delivered, but completely neutral tone of voice. It makes sense as it's a robot, but the way it says it makes it sound like assimilating everything non-organic into itself is as natural to it as breathing is to a human.
Not all languages have the same range of inflection, resulting in speakers of a language with a lower range, when learning a new language with a bigger range (for example Dutch speakers learning English), tend to sound monotome.
A bit of Truth in Television: behold flat affect, a common symptom of Post Traumatic Stress Disorder and the psychotic disorders (which includes schizophrenia).
Stephen Hawking speaks like this, though it's because of the synthesizer he has to use to speak at all. He's actually quite personable. His lectures certainly draw crowds.
It's not just Hawking. Anyone who speaks using an artificial voicebox has no choice but to use a monotone, as the mechanism doesn't allow for inflection and tone. Later, Hawking was offered improved software that could affect tone better and use a more appropriate accent; he turned it down because the synthesiser is recognised as being his voice.
Used by Robin Williams to show one of the ways you can find out if your neighbors have been taken over by aliens.
In an interview, Alfred Hitchcock explained that raving lunatics are not scary. He said it was the dead calm voice you can't reason with. He made a finger gun gesture and said in a dead calm voice "I don't want to do this, because my mother always said you shouldn't hurt people." Very creepy.