Some might be amused. For example, Thor's actor Chris Hemsworth might find it funny to see the wild adventures fans write about his character. He'd probably have a good laugh at the creative interpretations.
Some actors might be flattered by fanfiction. They see it as a form of extreme fandom and a creative expression inspired by their work. For example, actors from popular shows like 'Supernatural' have had a large amount of fanfiction written about their characters. Some actors may read it privately and enjoy seeing the different scenarios fans come up with, while others may publicly express their gratitude for the fans' creativity.
It can also be frustrating at times. Some fanfictions lack the proper understanding of the movie's lore. They might introduce concepts that don't fit well with the established Marvel Cinematic Universe. But on the positive side, many fanfictions are a labor of love. Authors pour their passion for the movie into their writing, and that can really shine through, especially when they capture the essence of the characters as we know them from the movie.
Well, one reaction could be excitement. Fanfictions often explore different aspects of the characters and storylines that might not have been fully delved into in the movie. For example, they might focus on the relationships between the Avengers in more depth, like the budding romance between Black Widow and Hulk. It can be really interesting to see how different authors interpret the characters' emotions and motives.
In this fanfiction, some Avengers might be in shock. For example, Captain America could be deeply saddened, his belief in justice and protecting the world shaken. He might feel a sense of failure for not being able to prevent the Snap.
Tony Stark might use his quick wit and technology. He could immediately scan Peter to make sure there's no physical danger causing the hyperventilation. Then he'd probably crack a joke to try and calm Peter down, like 'Kid, you breathe any faster and you'll fly away like a human jetpack.'
Some characters would likely be entertained. For instance, Loki might enjoy seeing the various ways fans write him as a more complex anti - hero. He'd probably find the creativity in the fanfiction quite appealing.
Thor would probably be the most enthusiastic. He'd see it as a great honor that the Midgardians (humans) are so invested in his story and his character. He might even want to join in on the fun and start coming up with his own ideas for fanfiction, thinking it's a great way to further his legend among the people.
Well, Thor might be a bit shocked at first. He'd probably be amused by some of the wilder interpretations of his brother in the fanfiction. Captain America, on the other hand, would likely be confused by the whole concept as it's so different from the traditional stories he's used to. Iron Man would probably make sarcastic comments about how creative the fans are in coming up with these stories.