@Outerspec:
Any change or development that happens in any story is for the sake of the plot.
BULLSHIT.
If you've got a disaster movie, or a super huge mega plot-twist the entire universe is built around, the story can guide the characters to a large degree with giant events. If the only thing the characters are doing is reacting to huge changes in the landscape, then yes, the plot is driving them.
But in pretty much any good story, the characters will drive it, no matter how epic the obstacle they need to deal with.
Plenty of stories take characters through twists, turns, loves, losses, and developments, some of completely out of their hands or unexpected (oh, Kefka blew up the world!) and it all seems natural and like a proper progression, and those big things will shape the plot yes, and what the characters do next…. but it shouldn't singlehandedly shape the characters.
Story elements get thrown at characters, and that's the plot. How they react and change to whatever random thing a story presents, is where the character comes in.
Tobidara threatens to destroy the world and Sasuke opts to team up with others to help stop him. Plot dictated twist, any character would be affected that way, including also Orochimaru.
Ultimate villain comes out of nowhere and after Sasuke has worked with his old friends, agreed ot be on the side of the angels, and sees that NAruto can do what he claims he do... he then decides for the fourth or fifth time to change sides again? That's not character development or growth or influenced by anything... other than the need to follow up on a final battle promise that had been in place for hundreds of chapters. (After all, his switch from teh darkness to goodguy was within a single page last time too. A single speech did the trick, not any sort of emotional journey or development.) SO it could be one more final fight.
I'm just going to cite Dragonball because everyone here knows it, even though I'm thinking of much better examples that I know no one has read or watched (I'm thinking Babylon 5, Shogun, West Wing, stuff like that.). I could write an entire essay on Vegeta and how you could see his incremental changes, how his constant humblings, death, losses, ego, family and desire to beat Goku worked at him, turned him evil again, then sacrificial, then willing to work with Goku, and so on... and they make sense.... but that's a long essay for elsewhere. Needless to say most of the big plot twists in later dragonball came because of Vegeta being himself... rather than adherance to the plot)
But for a very, VERY large chunk of Dragonball, Toriyama was writing towards the plot point of Gohan being uber strong, and someday surpassing his father. From almost the first chapter Gohan was introduced he was going to be stronger than all of them. He carried screen time and focus on Namek while Goku was out of the way for a long period, and during the Cell saga, Goku decisively handed over the reigns to him and Gohan became the strongest character. He was declared the defacto new lead of the series, the defender of the earth, the strongest by far.
Except. Gohan had never been a character that liked fighting. He wasn't built for it like the others were. And so, in the following arc, his training slipped, and he fell into third place. He had moments along the way where he manned up, did his job, and could have been the ultimate hero of it all... and driven sheerly by the plot, he should have been. ANd a lot of readers felt he earned that and should have. BUT. Toriyama felt Gohan just wasn't the ultimate hero type, and that Goku was. Debate however much you want on if it was the right move for the story or not... but Toriyama listened to the CHARACTERS. Gohan said he didn't want to be a hero, and Goku said he did.
So Toriyama wrote towards that instead. There was a plotted plan going on... but the characters spoke differently on what the outcome should be.
You have the president of the united states as a main character. And you want to explore what happens when he has a sick day. So you write the episode. But it has no punch or real point to it. And at the end of the episode, you reveal that he has MS. No plans or ideas or intentions of where that plot will go... but suddenly the characters said it was in their history, and it ran with it, shaping everything that comes after.
You try to assassinate a horrible emporer. You've got the embittered snarky old guard politician and his wide eyed innocent assistant. As the writer you fully plan for the old guy to pull off the assassination, but instead, the innocent ends up doing it... and suddenly that shapes him for years to come.
I'm a professional writer myself. I plot, scheme, plan events in advance. But sometimes, when you're doing it right, (and all great writers will say this) the characters speak for themselves. They go down paths you didn't expect, refuse to go down a plot point you've railroaded them into, and trying to write them that way feels wrong... and it often leads to more interesting paths than what had been planned, the characters are more organic that way, and you can see it in them.
Sasuke was plot railroaded the entire way. He was evil because the story demanded him and Naruto continue to be at odds... not because of any organic development, and his constant flip flopping was a sign of that. Hell, for a long while he was being written and described as if he had a literal evil demon inside of him controlling him, his darkness was so dark, complete with two page spread close up crazy eyes.
If Sasuke was going to react and change from what Itachi told him... that should have happened right after Itachi died. Not hundreds of chapters later after he continued to try and kill Naruto and Sakura and his teammates, then switched sides randomly in the course of ONE chapter, and then switched back again just a couple weeks ago so he could finish the fight he started in chapter 170.
Even IF he had proper and real development (and he didn't) if you look at the speech he gave in this latest chapter, its 95% about how Naruto made him feel... BEFORE THE TIME SKIP. Everything in the current fight was based entirely on that... how Naruto was someone he cared for, and felt envious of... the same stuff that motivated him before.
Nothing that was learned or came since! The only reason they had this fight is because they promised they would have a fight sequel 550 chapters ago, and the characters didn't grow or change enough for any possible other outcome. Even after working together again hours prior against a bunch of world ending threats and a bunch of revelations about the history of their world and genetic curses and all sorts of nonsense.
See also, Sakura continuing to be useless in a fight, AND lusting after Sasuke, AND cryign for Naruto to take care of him... even after he tried repeatedly to kill her. No growth or change from her either. Or Naruto learning or changing his mind based on experience either. He never went "Well okay, Sasuke really is a mad dog"... he's just been saying for 10 years "Sasuke is my friend based on things that happened off camera, and I'm gonna bring him back." No growth or change or new insights from anything that happened in the plot... just... the exact same characterization.