The biggest shift I've noticed happens when a character gets thrown into wildly different scenarios than the original story provides. Canon can be pretty rigid—character A is the hero, character B is the villain, their dynamic is set. But in fanfic, especially the kind that gets popular in anime fandoms, someone will flip the script entirely. Maybe the villain gets a redemption arc spanning 50 chapters, or the quiet side character becomes the central POV for a political thriller set in the same world. The original anime might only hint at a character's past; fanfiction builds an entire life around that hint, giving them motivations and flaws the source material never had time to explore.
It's not always for the better, obviously. Sometimes popular fanon interpretations flatten a character into a single trait—all tsundere, no substance—just because that's what gets the most kudos. But at its best, this process feels like collaborative myth-making. Thousands of writers adding layers, arguing in the comments about whether a certain action is 'in character,' testing different backstories. The character stops belonging solely to the original creators and becomes this evolving, communal entity. You start seeing the canon episodes differently, reading subtext into glances because a fanfic convinced you there's more going on.
That collective interpretation inevitably leaks back into official stuff too. Not directly, but you can tell when writers are aware of the fandom's deep dives. Maybe a future season spends more time on a pairing everyone ships, or gives a fan-favorite side character an extra scene. The fanfiction doesn't just develop the character on the page; it develops the audience's capacity to see more in them, which in turn can shape what gets emphasized later. It's a weird feedback loop.