Beware the Quiet Ones

"Shit's gonna go down when Fluttershy's voice goes above three decibels."

—YouTube comment on Fluttershy from My Little Pony Friendship Is Magic

"It's always the quiet ones, isn't it?"

—Slade, Teen Titans

When most people think of The Quiet One, they may also think Extreme Doormat or Butt Monkey alongside it. Whether they are one or not (half the time they are only seen that way), push them too far and they will give you a 'not so quiet' moment.

Alice seems to be The Quiet One who keeps to herself. Bob sees her as an easy target and makes her his honorary Butt Monkey to abuse. After weeks of quietly taking this, Alice snaps.

Maybe Bob pushed a certain button, maybe those anger management classes Alice used to take are starting to wear off, maybe Alice has been angry all this time, it just never showed or no one took it seriously. Maybe this is her first time dealing with anger.

No matter which reason, what comes after is the same: An unleashed raging or cold speech of epic proportions that not even the most demented character in the story would want to sit through.

This rage is always expressed verbally, though violence can also be included. Their verbal attack can differ from an ego-brusing "The Reason You Suck" Speech, a mentally scarring Breaking Speech or an emotional World of Cardboard Speech. If they are also The Stoic, they might also ask the offender "Did You Think I Can't Feel?" before ranting about all they have been put through. One might also catch a swear or two.

Another version could be when the Five-Man Band is in low spirits, and The Quiet One, fed up with all the sulking, throws the table (or something else) to the side and gives the Rousing Speech to their comrades.

Usually a Crowning Moment of Awesome when they are shown to finally speak up for themselves, but can change depending on the character or the setting.

The main difference between Beware the Quiet Ones and its sister trope Beware the Nice Ones is while the latter is known for their kindness, the former is known for their (near) silence, though they can overlap sometimes.

Compare and contrast Tranquil Fury, where the character can range from happy or stoic, but their anger is more quiet (but still dangerous).

Compare Dandere, Beneath the Mask, Shrinking Violet, Let's Get Dangerous. See also Rant-Inducing Slight.

Examples of Beware the Quiet Ones include:

Anime and Manga

Near-Invisible Canada from Axis Powers Hetalia is a good example: Butt Monkey? Check. Shrinking Violet? Yep! Giving America "The Reason You Suck" Speech? A true Crowning Moment of Awesome.
Machinedramon from Digimon Adventure is a villainous version. Don't let his whispered monotone fool you; he's as powerful and sadistic as any of the other Dark Masters.

Film

One of the things most notable about Matsu, the main character of Joshuu Sasori, is her knack for patiently and silently enduring whatever hellish mistreatment gets thrown her way and waiting for just the right moment to unleash violent revenge on those that wronged her. This is why she is called Sasori, or "Scorpion."
Easily one of the scarier examples of this trope is Gaear Grimsrud from Fargo. He hardly ever says a thing throughout the entire movie (something lampshaded early on by Steve Buscemi) but if provoked or put in a situation where he might get caught... well, let's just say that you don't want to be near him and a gun. Or an axe for that matter... or a woodchipper.
In Ghostbusters Walter Peck shuts down the power to the Firehouse despite Egon's dire warnings that it would be like dropping a bomb on the city. This results in an enormous explosion and hundreds (possibly thousands) of ghosts being released back into New York all at once. Peck orders some nearby police to arrest the Ghostbusters, claiming they were directly responsible for causing the explosion. Egon completely losses it, screams "YOUR MOTHER!!" and has to be physically restrained from tearing Peck apart with his bare hands. Remember, this is Egon Spengler!

Literature

In The Outsiders, Johnny was the most quiet and most law-abiding of the greasers but when he sees his friend Ponyboy being drowned, he stabbed and killed the attacker.
Dark Life has Pretty. Despite his name, he is not someone you want to annoy.

Newspaper Comics

In Peanuts, Marcie is a meek, quiet, mousy girl, and her patience level is high, but when someone is rude or nasty enough to deplete it (like Peppermint Patty's He-Man Woman Hater teammate Thibault did in one arc), WATCH OUT.

Web Comics

Finland from Scandinavia and The World has this moment, where he shouts Perkele 306 times after losing to Denmark in hockey.
Yuki Sonada from Megatokyo kind of fits this trope (more since her magical girl abilities awakened, than in her earlier appearances). That is, assuming she's not just being stereotyped as an over-emotional teenage girl.
- So do Piro and Ping, but if pushed over the edge they can be physically violent, too.

Western Animation

Raven from Teen Titans may be reserved, but god help you if you really piss her off. It doesn't help that she's half-demon, and destined to cause The End of the World as We Know It.
In the episode Bubblevicious, Bubbles is sick of being the overlooked sister and sets out to show how powerful The Quiet One can really be.
- Bubbles isn't really particularly quiet though. She's more of a case of Beware the Nice Ones.
Flora from Winx Club is sometimes thought to be a pushover because of her shy nature, unwillingness to harm living things and the nature of her powers (that pun wasn't intended; forgive me). But time and again, she proves that nature is not so harmless, especially in Tears from the Black Willow, when the witches hit her Berserk Button.
Fluttershy of My Little Pony Friendship Is Magic is the quietest member of the mane cast, on account of being, well, shy. Then comes "The Best Night Ever". When she's avoided by the forest creatures of Canterlot...well see for yourself.
- She displayed this way earlier, in Dragonshy and again in Stare Master. Dragonshy is the episode where the fans learned that you do not fuck with Fluttershy.
- Also, Celestia help you if Big Macintosh raises his voice.
The Transformers Prime version of Soundwave speaks using others' recorded sentances, rarely ever has a major starring role.. and can bash in heads with the greatest of ease, as Airachnid can attest to. He's the second-most feared Decepticon on the Nemesis, right under Megatron himself. Knock Out even says "Watch out for the quiet ones" to Breakdown after Soundwave curbstomped Airachnid.
In Transformers Animated, Prowl definitely qualifies for this. He's a sleek, dark ninja who doesn't really like anyone else, prefers to sit in his tree and meditate.. but when he fights, he wins, most of the time.