There are many possibilities. A character who is always looking for new knowledge or experiences might be very interested in the fanfic. They could study it to understand how fans view their world. On the other hand, a character who is more traditional might reject the fanfic completely, seeing it as a deviation from the 'true' story. And then there are characters who might be inspired by the fanfic to try new things in their own in - universe lives.
Some might be shocked. For instance, Morgana, who has her own complex story arc, may be taken aback by how fans rewrite her motives in fanfiction. She might not expect certain sympathetic portrayals.
Yuno might be a bit more reserved in his reaction. He's more of a calm and collected character. He would carefully analyze the fanfiction, looking at how the writer has portrayed his growth and his relationship with Asta. If he finds something that doesn't match his own view of his character, he might be a little disappointed, but overall he'd be respectful of the creative effort.
Rainbow Dash might see humans as potential rivals in terms of physical abilities. She'd be eager to show off her flying skills and might challenge a human athlete to a friendly competition. And if the human showed some really cool skills, she might gain respect for them. Pinkie Pie, of course, would just be all over the humans. She'd throw parties for them, introduce them to all her friends, and make sure they had a great time in Equestria. She doesn't really care about differences; she just wants to have fun with everyone.
They could also be confused. The fanfiction might take the story in directions that are completely different from what they know. Like the trolls might not understand some of the modern or human - centric concepts that fans include in the stories. Angor Rot, for instance, might be baffled by the way fans sometimes portray him as having a change of heart in some fanfictions, when in his own mind his motives are clear.
Well, it depends on the characters. For example, a more serious character might be critical of any inaccuracies in the fanfic. But a more laid - back character could enjoy the creativity. They might find it interesting to see how fans expand on the storylines that are only hinted at in the original show. They could also be surprised by the new characters or plot twists that the fanfic writers come up with.
In many fanfictions, Natsu's relationships are explored differently. If it's a fanfic about Natsu and his relationship with the guild as a family, then characters like Mirajane would be very nurturing towards him. In a fanfic where Natsu is on a solo adventure, the guild members would be eager to hear his tales when he returns. They might be excited, like Kinana, or skeptical, like Gajeel, depending on the details of the fan - made story.
A heroic type of character might see the Sans fight as a great challenge. They would be eager to face him and prove their worth. They might respect Sans as a worthy opponent and try to fight him honorably.
Some actors might be flattered by fanfic. They see it as a form of extreme fan love and creativity. For example, an actor from a popular TV show might find it interesting that fans are so invested in their character that they create their own stories. However, some actors could also be a bit uncomfortable with it, especially if the fanfic contains elements that are too far - fetched or inappropriate in their view.
Some characters might be critical. If they have a strong sense of the 'canon' Superman, they could nitpick at the details in the fanfic that don't match up. For instance, if the fanfic changes Superman's origin story too much, they might frown upon it.
They could be shocked. If the fanfic takes a really unexpected turn, like making a villain into a hero suddenly, the characters might be taken aback. It's like seeing a completely different version of their world.