Well, there are also those characters who might be a bit shocked or confused. Take Levi from Attack on Titan. He's a very serious and no - nonsense type of character. If he were to read some of the fanfic about him that deviated too much from his own self - perception, like being made into a comical or overly emotional character in the fanfic, he might be taken aback at first. But then he might also realize it's just a different interpretation and not take it too seriously.
Well, it depends on the nature of the fanfiction. Some anime characters might be amused if the fanfiction is light - hearted and full of comical situations. For example, if it's a fanfic about a cooking competition among the characters from a shonen anime, they might react with good - natured laughter and competitiveness. They could also be confused if the fanfiction takes their story in a very strange or unexpected direction, like suddenly making a character from a historical anime end up in a space adventure.
Sakura would probably be both scared and fascinated. She might initially be frightened by the dinosaurs but then use her medical ninja skills to help any injured people. Sasuke would be more aloof. He'd likely see the dinosaurs as just another challenge. He might study their behavior to find weaknesses and use his sharingan if any of the dinosaurs pose a real threat.
Ladybug would see Disneyland as a place full of positive energy. She would be cautious at first, always on the lookout for any signs of trouble. But as she explores, she'd start to enjoy the different shows and parades. Chat Noir would be like a big kid. He'd want to try every single ride and interact with all the costumed characters. He might even challenge some of the Disney villains to a friendly duel. And Marinette, well, she'd be busy taking mental notes for her future fashion creations, while also being in love with the whole Disney atmosphere.
Kakashi would be very observant. He would look for how the fanfiction writers understand the world of Naruto, the values and the moral lessons. He might even use some of the creative ideas from the fanfiction in his own teachings if he thought they were valuable and in line with the spirit of the original story.
There could be characters who are outraged. If the fanfiction distorts their personality or makes them do things completely against their nature, they may be very angry. For example, a noble character made to act like a coward in the fanfiction would likely not take it well.
They might be confused at first. RWBY has its own unique lore and concepts that are different from many anime. So anime characters may need time to understand things like Grimm and Huntsmen. For example, if Goku from Dragon Ball was watching, he'd probably be like 'What are these strange monsters?'.
They could be surprised. The fanfiction might show them in scenarios they never imagined, like a shy character being in a very bold and adventurous situation.
He could use it as motivation to improve. Maybe there are aspects of the novel that he can work on, like the marketing or the plot development. He could also try to engage with the people who made the comment in a polite way to understand their perspective better.
They could be angry. Consider a character like Ichigo from Bleach. If the fanfiction makes him look weak or changes his character in a way that goes against his core values, he would likely be furious. After all, he has his pride as a Soul Reaper and a protector.