Mulan II derails several characters: in addition to giving Mushu Aesop Amnesia and making him a bigger jerk, Mulan and Shang start arguing by using sex stereotypes as insults, and Chien Po goes from a Gentle Giant who loves to eat to a flatFat Comic Relief character who is Obsessed with Food (and engages in pointless brawls, something he pointedly did not do in the first movie). But the character who got it worst was Ling, whose personality got traded in for... wanting someone to laugh at his terrible jokes. This becomes his motivation, despite not even being hinted at in the first movie.
Pocahontas II: Journey to a New World derails many characters as well. Pocahontas herself became cold and distant, John Smith was changed into a wise-cracking self-absorbed Jerkass, Meeko fell victim to Aesop Amnesia (especially in regards to his friendship with Percy) and Ratcliffe's evilness got worse.
Titan A.E. manages to derail a character within one film. When Korso is revealed to be a Drej agent, the charismatic and competent captain of the Valkyrie is replaced with a bullheaded brute who makes rash decisions and is outwardly antagonistic towards Gune and Stith. Even when he makes a Heel–Face Turn, it's still such a drastic change in character that you wonder why Gune and Stith hadn't cottoned on sooner.
Considering all of its faults, many agree that one of the greater atrocities committed by The Secret of NIMH 2 was entirely derailing the character of Brutus; changing him from a white-eyed, silent, spike-wielding gatekeeper into...a blundering, idiotic side-kick to the also-somewhat-derailed Justin.
The original book Mrs. Frisby and the Rats of NIMH did actually reveal Brutus to be a pleasant guy just playing the shadowy guard act to scare away intruders. Similar to the Aunt Shrew example, it is more the fault of both films for not elaborating on this enough.
Or Auntie Shrew scolding Martin for badmouthing Nicodemous and insisting that he was a great oracle or something - despite the fact that she clearly was shown in the first movie distrusting the rats (though it's possible she thought better of them after they helped the Brisby family, there really isn't any mention of this).
Journey Back to Oz: This is done to Dorothy's three friends from the original movie. In The Wizard of Oz, despite physically lacking a brain, a heart, and courage, the Scarecrow was actually quite clever when he put his mind to it, the Tin Man was very caring when it came to the well being of others, and the Lion could be counted on to conquer his fears when the chips were down and the others needed him. Here, however, the Scarecrow is a totally useless idiot, the Tin Man cares more his own well being than anyone else's, and the Lion is now a total wimp scared of his own shadow. How they still have kingdoms to run at the end of this film, despite contributing pretty much nothing to the defeat of Mombi, is a question for the ages. At least the songs they got to sing were catchy.
Films — Live-action:
Halloween: The Curse of Michael Myers: Michael Myers was known as an iconic soulless killing machine who can't be intimidated, bargained with, reasoned with, or stopped, and whose only explicit motive was to kill his family. In the director's cut of this movie, he rapes and impregnates his niece on someone else's orders.
Kick-Ass 2: Katie Deauxma is an extremely irritating offender of this, having lost her very pleasant and outgoing demeanor from the first film all of a sudden in this one. While it could be understood that boyfriend Dave Lizewski’s time as Kick-Ass has been pushing them apart between movies, Katie immediately accuses Dave Lizewski of coming onto Mindy Macready without any proof based solely on an out of context conversation between the two as she should probably know him well enough to know he wouldn’t do that. Also, when Dave attempts to explain that it had to do with the superhero stuff, Katie right out of nowhere admits to cheating on him and then never shows up again for the rest of the movie.
John Kramer, better known as the "Jigsaw Killer". Even though he's always been considered a Serial Killer, he made a huge point that he "never killed anyone", merely putting them in position where they had to fight and occasionally compete to live. Then from Saw V onwards (well after his death in Saw III, though he still had some posthumous presence), he suddenly was fine with letting Hoffman use his traps, which did kill people outright.
They didn't kill people outright any more than Amanda's trap in the very first film did, or Lawrence's test as intended. Come Saw 3D, he has no real such excuse, as the final trap is impossible and kills Joyce out of spite rather than as a result of Bobby's own failure. Add this on top of Lawrence's example below, Jill's criticized Chickification (despite it being justified in-universe), and Hoffman's Flanderization into an Ax-Crazy slasher villain, and every returning character got derailed in this movie.
It's worth remembering that John did wound a police officer (David Tapp) with a concealed dagger in the first film, and there was no way that a man just doing his job could have "earned" his survival from that attack. His moral stance can seem more of a pretense for his actions rather than a motive when you take that into account.
Lawrence Gordon's long-awaited return in Saw 3D counts as this too. To wit:
This is considering the last time we saw him (in the first film), Lawrence was crawling away from the Bathroom, shivering from the blood-loss incurred from cutting off his foot and promising he'd find help for Adam, the still-shackled guy being tested with him (this is after he believes his family may well have been shot and killed due to his inaction in the "test").
In Saw 3D, flashbacks show that he became a willing accomplice to John after the latter nursed him back to health when he attempted to cauterize himself with a boiling pipe and fell unconscious in the process. This could have been acceptable, if not for the fact that in the first scene we see Lawrence in during the present, he delivers a cheesily villanous-sounding monologue to Bobby, which greatly contrasts with the distrought husband/father he had become throughout the first film. Whatever real change occurred apparently happened completely offscreen, and what's more, apparently happened in the span of about two months, as we see Lawrence smiling whilst placing a key behind Michael's eye in a flashback to Saw II. At no point in Saw 3D does he mention his family, his affair Carla, or Adam, and he only looks at the latter's corpse rather dully in the climax when he returns to the Bathroom; this is still pretty weird with Word of God saying that Lawrence was still going to save Adam after his recruitment, before Amanda got before him and performed a Mercy Kill on Adam.
Purposely done in the case of Susan Orleans in Adaptation.. Portrayed as simply a reporter who develops a bond with John Laroche, due to Charlie Kaufman's in-story Creator Breakdown she proceeds to become a psychotic drug-addict who regrets her life.
Space Jam does this to Bugs Bunny. Usually, he's a laid-back, sarcastic, mischievous Karmic Trickster who'll stand up to anybody - no matter how much bigger they are - if they antagonize him and is always able to outsmart them. In Space Jam, meanwhile, he's a pathetic loser who's actually afraid of, and outsmarted by, a group of tiny dimwitted aliens and needs the help of the other Looney Tunes characters and Michael Jordan to defeat them. Even when Bugs was afraid of his opponent in the original shorts, he would never beg another character for help. Not only that, but at one point he actually needs to be saved from a dog. That's how pathetic Bugs is in this movie. No wonder Chuck Jones hated the film.
Unfortunately, Falcor also got this treatment. He was initially a wise, gentle creature that would gladly lend a helping hand, but in this one, he's the dragon equivalent of Scooby-Doo.
Michael Corleone. Compare this character between of The Godfather Part III and the rest of the series. In the first two parts he was a dead-serious and moody fellow. In the final sequel he's got cute quirks and snarks.
Heisei Rider vs. Showa Rider: Kamen Rider Taisen feat. Super Sentai is an infamous example among Toku fans. Unlike most "Versus" movies, this isn't a fake-out; the Showa Riders have a major quarrel with their juniors, attacking them while Badan raises a literal undead army. And even after Badan is defeated, they still insist on fighting the Heisei Riders. What's their beef with the younger heroes, you may ask? The fact that they remember and honor their deceased friends. Ignoring the fact that this makes the Showa Riders look heartlessnote or how it makes them look stupid; the Heisei Riders' attachments to their dead friends helped Badan bring back dead enemies, but they didn't bother explaining this until after Badan was defeated, instead jumping straight into attacking the Heisei Riders, it completely ignores the fact that practically EVERY Kamen Rider has lost a friend or family member somewhere along the line (creator Shotaro Ishinomori envisioned them as "crying heroes"). The Derailment actually bothered Hiroshi Fujioka (the star of the original Kamen Rider, who returned in Heisei vs. Showa) so much that he ended up co-writing his own movie to do justice to his character.
Original solicitations for the film suggested that there was a different conflict: the Showa Riders prioritized defeating evil, while the Heisei Riders prioritized protecting the innocent, as seen with their helping a young boy say goodbye to the spirit of his dead mother. While this is still Derailment (since Kamen Riders are all about protecting humanity's peace and freedom), it's not nearly as bad as what we got in the final movie.
Godzilla vs. Kong: Mark Russell had a character arc in Godzilla: King of the Monsters (2019) wherein he went from hating Godzilla to forgiving him, but in GVK, he doesn't even bother to figure out why Godzilla's out there hurting people.
Pirates of the Caribbean: Dead Men Tell No Tales: Captain Jack Sparrow is no longer that smarty and cunning type of character, but rather just a drunk dupe, with no unexpected moves whatsoever. He just goes with the flow of the plot, with having little to no impact on it. The highest point of Jack's derailment was when he fell into the pigsty and then he casually bartered the most important artifact of his life - the compass, for what? The bottle of rum. Possibly, it is the stupidest thing he ever did in the entire franchise (to make matters worse, he actually had a gold coin that he received earlier in the movie). He's persistently saying 'Pirate's Life' throughout the movie like someone who is just giving up.
Loki got hit with this in The Avengersthe moment he picked up the Mind Stone, going from being a trickster with a treacherous streak who nonetheless cares deeply for his adopted family to being a mad fascist hellbent on taking over Earth beginning with the Big Apple. He's ultimately rerailed beginning in Thor: The Dark World, where he returns to being the habitually-treacherous trickster and a more sympathetic figure overall.
Avengers: Endgame: Steve Rogers deciding he'd rather stay in the past is infamously one of the most hotly divided aspects of Avengers: Endgame due to being considered an example of this. Many fans felt that his subplot in this film about his overwhelming desire to reunite with Peggy is a hasty about-face after previous films placed a lot of emphasis on his desire to adjust to the modern era and his efforts at finding a Second Love in Sharon Carter, who is inexplicably absent from the film, and after every past appearance of his focused on his efforts to rescue and help his best friend Bucky, whereas in this film he makes the decision to leave the now-heartbroken Bucky completely offscreen.
Ian Malcolm's cameo in Jurassic World: Fallen Kingdom has him echo his "what can go wrong, will go wrong" rhetoric from the first movie, but the script doesn't seem to understand the point behind it. Notably, Malcolm ominously states at the end of the film that the escaped dinosaurs could overrun the planet, something he furiously lambasted Hammond for believing in the novel.
Scooby-Doo (2002): In the original cartoons, Scrappy was an impulsive, yet well-meaning puppy who admired the Mystery Gang, adored his Uncle Scooby (even thinking he was the leader of the group) and did all he could to help. Not to mention he was insanely strong and durable. In the movie, he's depicted as an arrogant, selfish, evil mastermind who got thrown out because he wanted to be the leader and accidentally urinated in Daphne. Even when defeated, he tries to fight Scooby, but gets smacked aside, practically helpless without his mech suit or the potion that made him huge.
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