In the fanfiction, a character might also react with confusion. Sans' attacks and his overall fighting style can be very unique and hard to understand. A character who is used to more straightforward combat might be completely bewildered by Sans' moves, not knowing how to counter or even react properly.
Frisk could be one of the characters watching. Given their role in the game, they would be interested in Sans' fight as it could impact their journey through the Underground. Additionally, Undyne might be present. She's a strong fighter herself and would probably be sizing up Sans' techniques, maybe even learning from him or being critical of his fighting style depending on her mood.
Surprise. They might be surprised to see how fans imagine different scenarios for them. For example, a character from a magical girl anime might be shocked to find a fanfiction where she's suddenly a detective in a noir - style story.
In a fanfiction, if Naruto kisses Kaguya mid - fight, many characters would have complex reactions. For instance, the other Kage from different villages would be appalled. They see Kaguya as a threat to the entire ninja world. Naruto's action could be seen as a betrayal. Hinata might be heartbroken. She has deep feelings for Naruto, and this sudden act towards Kaguya would be a huge blow. The Akatsuki members, if they were still around, would probably see this as an opportunity to use the chaos to their advantage. They might try to manipulate the situation to further their own goals while everyone else is distracted by Naruto's strange behavior.
One interesting thing could be the different reactions of the characters. For example, some might be in awe of Sans' combat skills, while others could be worried about the outcome. They might cheer him on or try to interfere depending on their relationships with Sans.
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 Twilight has a very unique world and characters, seeing different interpretations in fanfiction could bring a sense of novelty and humor to them.
Some anime characters could be confused. Consider a more serious and by - the - books character like Levi from 'Attack on Titan'. Fanfiction often takes creative liberties and goes against the established canon, so he might be really confused by the strange plotlines and character interpretations he reads in fanfiction.
They could also be offended. If the fanfiction portrays them in a way that goes against their values or character traits in the original work, they may feel misrepresented. For example, if a character is always shown as noble in the original but is made into a villain in the fanfiction. They might not like this change at all and feel insulted.
They might be amused. Since fanfiction often contains creative and sometimes humorous takes on the original story, characters could find it funny to see how fans imagine different scenarios for them.
Harry Potter might be a bit surprised. He'd probably think it's strange that people are writing about his friends' and his own relationships in such a different way. Hermione would likely be very interested in the writing style and how the authors are creating these new relationships. She might even take notes to improve her own writing. Ron would just laugh it off most of the time. He'd find it a bit odd but also funny.