The characters could have a variety of reactions. First of all, they may find it interesting to see how fans view their world and relationships. A character like Johnny Blaze (Ghost Rider himself) might be intrigued to see how fans imagine his internal struggles in the fanfiction. On the other hand, supporting characters might be surprised at how much focus they get in some fanfictions. Some might even be inspired by the new ideas presented in the fanfiction and might start to think about their roles in different ways.
They might be shocked at the creative interpretations. Some characters could be amused by the new and wild storylines that fanfiction often brings. Others might be a bit offended if the fanfiction takes their character in a direction they don't like.
They might be surprised at how creative the fans are. Some characters could be amused by the different interpretations of their personalities and the events in Endgame. For example, if Iron Man's character reads a fanfiction where he has a different ending in Endgame, he might find it interesting to see how fans envisioned an alternative scenario for him.
They might be amused. Since fanfiction often takes the original characters and puts them in new and sometimes humorous situations. For example, the serious Allen Walker might find himself in a comical scenario in the fanfiction, which could make him crack a small smile if he were real and watching it.
They might be amused. The Grinch has such a unique character, and fanfiction often takes his story in new and unexpected directions. So the characters could be laughing at the humorous twists and turns in the fanfic.
It's interesting because it allows fans to explore different scenarios that might not be in the original movies or comics. For example, new relationships for Ghost Rider or different ways his powers could be used.
Others could be really intrigued. They might get caught up in the new storylines and start to wonder what if these things could actually happen in their universe. Like Arnold, he's always open - minded, so he might be really into exploring these new fan - created scenarios.
Hero characters might analyze it from a moral perspective. They could either approve or disapprove of the way the story is told in the fanfiction depending on whether it aligns with their sense of right and wrong. Villain characters, on the other hand, might look for ways to use the ideas in the fanfiction to their advantage, like new ways to create chaos.
They could be shocked. Seeing the strange and dangerous world of Lyoko for the first time might leave them speechless.
Some of the Hetalia characters might be offended. If the Hamilton fanfiction distorts or misrepresents certain historical events or figures in a way that goes against their understanding or values related to history, they could be put off. For instance, if a fanfiction portrays a historical event in a way that seems disrespectful to their 'country', they would likely not be happy about it.
Glee might be excited. Fanfiction often has some really cool and unexpected twists that can make for an exciting read or viewing experience. They could be really into the new takes on the Flash's powers or the relationships between the characters.
They might be amused. Since fanfiction often takes the original story in new and unexpected directions. For example, if a character like Peter Parker's friend Ned was watching Spiderman fanfiction, he might laugh at the creative interpretations of Spiderman's adventures.