Archie Meets the Punisher ends with The Punisher sparing the villain in an uncharacteristic display of mercy. He reasons that killing him would utterly destroy Riverdale's innocence, and he isn't willing to go that far.
Asterix: Subverted in "Asterix the Legionary", where Asterix is worried that something is wrong with his best friend Obelix due to him losing his appetite when he's normally a Big Eater. As it turns out, however, there's nothing wrong with him; he just has a crush.
Skalman from Bamse, a Swedish comic series, almost never showed emotion, and always obeyed a strict schedule. When he stopped obeying that schedule for a bit, or snapped at his friends, you knew it was serious.
In the comic book adaptation of Darkwing Duck, the Liquidator, who has the Verbal Tic of speaking in ad slogans and bogus claims, suddenly drops it just long enough to warn someone about Quackerjack's Berserk Button. Which turned out to be Serious Business indeed.
In the 3rd book of Bone, while the world slowly crumbles due to the Rat Creatures and Thorn learning she is a princess, Grandma Ben, whose eyes are always closed, opens them for a split-second when she finds out that she has partially doomed the Valley by not telling Thorn that she is a princess.
The Caged Demonwolf in Empowered drops his alliteration and wordiness at one point for a genuinely touching discussion with a character about how he'll always remember her. After five or six volumes of "Bah!" it's an attention-getting moment.
In the third issue of IDW's Ghostbusters: Mass Hysteria (issue 15 of the 2013/2014 series), Peter takes a phone call for help while discussing the current problem with Walter Peck and immediately leaves via motorcycle without a word, much to Peck's surprise. Considering the caller was Dana Barrett, it's understandable.
Walter Peck: Ms. Melnitz, after taking that phone call, Peter Venkman ran out of here without a single smart-assed remark. That kind of character inconsistency is, in my experience, the reddest of red flags.
Vladek is normally a boisterous and obnoxious old man. So Art is taken aback when he finds Vladek treating him coldly. Art learns that Vladek read the comic book he wrote about his mom's suicide.
During his time in Auschwitz, Vladek comes to know one of the guards, who's rather friendly for a Nazi. One day, however, the guard is absent; he returns the next day, extremely depressed and quiet. When Vladek asks if he's ill, the guard simply replies that he was assigned to Birkenau the previous day.
Be wary if Donald is angry but keeping his anger in check: whatever he's planning to do is much worse than what he'd normally do in his usual fits of Unstoppable Rage. This was shown at its best in "Donald and Reginella's Wedding", where he kept his anger in check the entire time while wielding a double-barreled shotgun with plenty of salt shells, and by the time he was finished with Bingo and his army they were so broken they accepted to melt down all their weapons and turn to agricolture for fear of him finishing to wreck them (because he wasn't going to give them the sweet release of death).
Paperinik stories (both "classic" and Paperinik New Adventures ones) that feature Donald angry are pretty much based on this idea, because Donald as Paperinik never goes in a fit of rage. The moment you should just start praying is when he stops wisecracking or shouting, though what he's going to do depends on how he acts:
If he looks bored, it means he's just too annoyed at you to joke and is now going to beat the crap out of you. A good example is in the story where professor Brain wouldn't reveal where the commands for his time-manipulation device were hidden or what the unlocking code was, Paperinik and his future counterpart looked bored, and the next panel a very beaten up Brain was spilling the beans. Once the timeline is restored with an alteration that prevents the device from working he starts looking bored right after the device fails and he's attacked by Brain's mooks - we don't see what Paperinik does them, only that Brain is horrified at the Mook Horror Show and when he's done the mooks are piled up in extreme pain.
If he's giving you a psychotic smirk, run: he's going to make you suffer if not outright kill you, you and whoever is stupid enough to try and defend you. This is shown best in "Paperinik and the Duel of Mages", where he's smiling almost the whole time as he hunts down every single charlatan and fake wizard in Duckburg to unmask them in the most public and humiliating way, fully knowing he's ruining their lives and slowly getting the people of Duckburg to form a lynching mob against them - and he's still smiling as the last fake wizards still in town are being chased by said mob.
Another alternative when he gives a psychotic smirk is that he's bluffing to get something from you, as he did once in the Paperinik New Adventures reboot - or maybe not. What is known is that he was smiling as he smashed aside Vendor's Mooks, announced he was about to kill him, aimed a laser cannon at his head and pulled the trigger, with only Lyla pushing the cannon aside and saying there was a way to fix what Vendor had done and they needed him for it apparently saving the Mad Scientist - who promptly followed suit out of fear of the superhero "changing his mind" again.
Miklos' entire shtick is to use his disguise skills and resemblance to Mickey to pull crimes and frame the latter for them, usually in a plan to eventually try and take over the latter's life and make it appear Mickey is the imposter - something that, quite understandably, makes Mickey want to react with violence the moment he sees him. But in "The Tree of Truth", the final story of the "Gambaville" miniseries, Miklos breaking out of jail, going to Mickey, and claiming that not only in their last encounter he didn't try it but asking for his help to find the actual culprit was so absurd Mickey gave him a chance to explain himself and prove his claims. Miklos being smart, he had brought all the evidence he had, and his mention of Scuttle, Pete's usual accomplice, helped him by triggering Mickey's well-motivated paranoia toward his usual adversary in spite of the latter having recently reformed. Miklos was sincere and Pete was indeed behind everything while pretending to have reformed.
When the Cutie Mark Crusaders aren't interested in cutie marks, you know something bad has happened.
Scootaloo turning down an offer to hang out with Rainbow Dash is so upsetting that RD has to tell her friends about it. (Whether she's worried for Scoot, is just nursing her bruised ego, or both is uncertain.)
The Fall of Sunset Shimmer has the normally calm and patient PrincessCelestia continually aggravated as the comic goes on, and even snapping at her student at one point. Not that it's unjustified since Sunset was being defiant, and all Celestia was really doing is refusing to give in to her behavior.
In Issue 25 of Pocket God, the ditzy Nooby is at the mercy of his Evil Twin, Newbie. When things seem dire, Nooby drops his usual third person speak and talks in articulate sentences to level with his more intelligent twin. The sudden change shocks everyone and it really gets under Newbie's skin.
Nooby: Nooby knows how it feels to not be accepted. Noob... I know what it feels like to be the odd pygmy out. Ooga: He's making sense?!?!
In a Simpsons comic story, Bart trades places with Biff Westwood, a celebrity who looks like him. He traded places the night before a test, and Biff studies for it, scoring an A on it. Later, Marge takes "Bart" to a meeting with Principal Skinner about the "A" he got, with Skinner saying that Bart is, was and always will be an underachiever.
During the final battle between Sonic and Robotnik in Sonic the Hedgehog (Archie Comics), Sonic is so pissed off about Robotnik seemingly killing Sally and destroying his home, that he doesn't quip any jokes and even refers to Robotnik by his real name. The comic itself lampshades this.
The fourth issue of The Tick comic book, "The Night Of A Million Zillion Ninjas" seems much like the previous chapters, until The Tick's friend and partner Oedipus gets stabbed by two ninjas, seriously enough that she falls unconscious and starts losing blood. At this, the wheels come off — rather than his usual bombast and style, The Tick quickly, silently, and violently dispatches the two ninjas, and all he can say as he carries Oedipus is "This isn't supposed to happen." He barely registers the paramedics who come to help Oedipus, and is later seen with paranoid delusions of the various buildings taunting him over his failures. Unlike most Cloudcuckoolanders, he doesn't suddenly become sane... he just stops being the "fun" kind of insane.
Star Trek (IDW): In Countdown to Darkness, Kirk is nearly lost for words when Spock apologizes to him for putting emotion before the needs of the mission.
Invoked and weaponized in The Transformers: Dark Cybertron — it takes an incredibly irrational and out of character act to breaks Shockwave's logic circuits and snap him back to his pre-empurata self and that's Megatron denouncing the Decepticon cause and joining the Autobots.
The Transformers: More than Meets the Eye: Whirl is an ex-Wrecker and Blood Knight of the highest order, regularly boasting of his status as an unstoppable force on the battlefield. But then he and Swerve run into the Whirl's Arch-Enemy Killmaster, at which point Whirl freaks out, orders Swerve to get help, and immediately after admits he's never beaten Killmaster. As if that wasn't enough, when said help arrives in the form of Cyclonus, the bot who Whirl has had the most vitriolic relationship with, Whirl's actually relieved.
In White Sand, when a known Determinator Kenton gives up, you know the situation's serious.
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