The long form of the trope title is "Out of Character Is Serious Business."
Some characters have strong traits that they are known by. This is for when characters momentarily break away from their normal habits to make a point about the seriousness of the situation. Often causes the other characters to do a Double Take and mention why this event is Serious Business. When most or all of these OoC moments happen at once, you can be sure that the Darkest Hour has arrived, leading characters to behave in ways they normally wouldn't, because they know they might not have another chance to do so.
It's also a pretty good indication the Godzilla Threshold has been crossed.
This is a trope for when a somewhat-Out of Character action is used to draw extra attention to the scene (similar to a Title Drop). It isn't Hidden Depths because it isn't telling us something about the character we didn't already know; it's similar to an Out-of-Character Moment in that this is specifically the usage of such a moment to draw attention to a scene. If they're doing this deliberately to make another character think something is amiss, see Out-of-Character Alert. Compare Let's Get Dangerous! and Weirder Than Usual.
A Super-Trope of:
Alternate Catchphrase Inflection: Someone says their Catchphrase in a different voice than usual.
Batman Grabs a Gun: Characters do something that breaks their own rules.
Beware the Nice Ones: When a character that's usually nice and patient ends up flying into a rage after getting sick of putting up with a less nice person's crap.
Beware the Quiet Ones: Characters who don't speak much turn out to be more threatening than they led on.
Beware the Silly Ones: Characters prone to silly and bizarre antics proves that their silliness doesn't mean they're harmless.
Moment of Weakness: A normally sensible character makes a devastating lapse in judgment.
Not So Stoic: The normally emotionless character expresses emotion when things are serious.
Offending the Fool: A normally ditzy or stupid character realizes that a line has been crossed, and call out the offendee.
Rage-Breaking Point: Characters have repressed their anger and eventually let it all out.
Stutter Stop: A stuttering character loses the speech impediment during a serious moment.
Sudden Principled Stand: Characters with few scruples suddenly refuse to follow an order too unforgivable even by their standards.
Suddenly Speaking: A silent character suddenly starts speaking.
Trivial Tragedy: When not Played for Laughs, a character bursting into tears over a minor inconvenience is a sign that they are suffering from a lot of stress, and the minor inconvenience is merely The Last Straw.
Verbal Ticked: A character with a Verbal Tic speaks normally because things are getting serious.
We Want Our Idiot Back!: A stupid character behaves smarter but everyone else wants them to be stupid again.
We Want Our Jerk Back!: A Jerkass or Jerk with a Heart of Gold starts acting nicer, but everyone else wants them to be mean again.
You Called Me "X"; It Must Be Serious: Characters realize they're in trouble when someone else addresses them in a way they normally don't.
Often overlaps with:
Appetite Equals Health (OOC if someone who's usually a Big Eater rejects food, or if someone rejects a food they usually like.)
Captain Obvious (OOC if a character who is normally too smart and sophisticated to talk down to their peers or associates without looking unprofessional, foregoes the façade in order to get to the point immediately. The same can apply to a character who conversely starts of as too oblivious to figure something out until later on. This can also overlap with Punctuated! For! Emphasis! so that they are loud and clear with their point.)
Dangerous Drowsiness (OOC if the afflicted character is normally energetic.)
Even Evil Has Standards (not OOC if those standards pop up often enough that the character is established as a villain with a few objections, but it is OOC if that character previously didn't appear to have standards)
Everyone Has Standards (OOC if the persons call out someone else on a fault that's no different from their own vice)
Explain, Explain... Oh, Crap! (OOC if a character is explaining midway something that seems innocuous at first until putting it into words under a specific context, gives them a belated, shocking realization about the thing they were talking about, and changes the context and/or true weight of the issue from their perspective, thus leading to a change in attitude from apparent ignorance to Serious Business on the spot.
Forgets to Eat (OOC if they're normally a Big Eater)
It Makes Sense in Context (how circumstances are arranged for some people to do what they do)
Old Master (who doesn't break into fights for just anything)
Precision F-Strike (OOC if the trope applies because the swear was unusual for the character, but not an example if it was just unusual for the work)
Prelude to Suicide (a character acts either unusually chipper or somber right before attempting suicide)
Sarcasm Failure (the Deadpan Snarker tries to make a dry quip, but can't; not OOC if someone who's not a deadpan snarker can't make a quip)
Silence of Sadness (Not OOC if they're usually quiet or talk an average amount, but OOC if they're talkative)
Suddenly Shouting (if the "sudden shouter" doesn't normally shout)
Suspicious Missed Messages (if the unresponsive character is normally addicted to their phone/computer)
Too Desperate to Be Picky (if characters violate their own set standards in an act of desperation)
Too Spicy for Yog-Sothoth (a potential victim is too much for the all-powerful being of darkness to consume or harm)
Too Unhappy to Be Hungry (Not OOC if they have a generic palate, but OOC if they're a Big Eater or they really like the food they reject (e.g Trademark Favourite Food, Sweet Tooth, etc)).
Too Upset to Create (if a normally highly-creative character is too upset to make anything for once)
Tranquil Fury (if the characters normally are not angry or express their anger)
When She Smiles (a character who usually doesn't smile starts smiling)
Working Out Their Emotions (for characters who aren't usually athletic)
You Are a Credit to Your Race (for racists)
