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.
"Yes, I am fine. I'll be even better once I load my gun, though." The character has just suffered a Heroic BSoD or one too many rounds of Break the Cutie and/or is about to go completely homicidally berserk. Usually paired with jittery motions, twitching, or a Broken Smile.
"Hello, do you have some raw meat?" One of the hero's friends or family has been Demonically Possessed, Brainwashed, or mind controlled via The Virus and are exhibiting Virus-Victim Symptoms. Usually, they're normal, except for odd behavior.
"Good morning annoying sibling, I am your sister Melanie." An evil duplicate (Robot, Clone, Shapeshifter, Doppleganger, or what have you, even zombie/ghost possession/mind control) has replaced the character. Their tell will usually be not knowing names and facts about their host and friends, knowing too much about their host, assuring everyone that they are Most Definitely Not a Villain, or a radically different personality. May be Too Dumb to Live at things the real one — or even anyone without too much Fish out of Temporal Water and minimal Genre Blindness — could do perfectly fine. A protagonist doing this, impersonating the hero while the hero is doing something else, is almost inevitably going to be the one that will be the most idiotic and provide either Fanservice or tons of Squick and Fan Disservice.
"Yes, dear. I'm fine. Please bring the MacGuffin to the Haunted Castle, won't you?" Hubby is being coerced by force to trick the hero into a trap, often includes an Out-of-Character Alert to try cluing the hero in to their being kidnapped. "How is our pet poodle Muffin?"
"I am sorry. If it's any consolation, this likely won't hurt much." If a character is introduced speaking like this, 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., Sociopath, Ax-Crazy, or some other seriously dangerous character.
"Hello! My name is Sally! It is very nice to meet you!" A case of Vocal Dissonance leads people to think her naturally frigid, emotionless, unfriendly voice means she's evil or at least an Emotionless Girl but she's actually a very warm-hearted, friendly, and kind person. She just happens to have a voice that can turn your blood to icewater instantly. People's first perceptions become a running gag.
Machine Monotone is a Sub-Trope. You may wish to compare Dissonant Serenity. Can make a very effective Deadpan Snarker. Compare Tranquil Fury, Alternate Catchphrase Inflection, and Voice of the Legion.
