She'd be appalled. Umbridge is all about following the rules and maintaining the status quo. Fanfiction doesn't adhere to the official canon she so firmly believes in.
She'd probably be horrified. Fanfiction can have all sorts of wild ideas and interpretations that go against her traditional views. For example, if there are relationships in the fanfiction that she doesn't approve of or characterizations that are different from what she thinks is right, she'll be very unhappy.
She'd be disgusted. Umbridge is all about propriety and order. Fanfiction can be wild and creative, which is the opposite of what she stands for.
He'd probably be furious. Voldemort has a huge ego and most fanfiction would likely not portray him in the way he sees himself. He might view it as a form of disrespect to his 'true' nature and power.
They would probably be very curious at first. But as they read more, they might get annoyed if the fanfiction doesn't stay true to their essence. For instance, if a fanfiction portrays Sirius as a coward, which is completely against his real character, they would be offended. However, if they find a well - written fanfiction that respects their characters, they might enjoy it.
Well, some of the cast might be really intrigued. They would see how the fans have filled in the gaps or created alternative universes. However, they could also be a bit critical. If a fanfic misrepresents a character's values or relationships that were carefully crafted in the books, the actor might feel a bit disappointed. At the same time, they might also be in awe of the fans' creativity in coming up with entirely new storylines and scenarios.
She would be appalled. Fanfiction may have different versions of characters or events that are not in line with her strict understanding of the wizarding world. She would likely try to suppress it.
He could be shocked. Maybe there are some fanfictions that take a very dark turn or have unexpected pairings that he never thought of. He'd probably need some time to process such strange ideas. And he might also be curious to see how far the creativity of the fans can go. He might even get some new ideas about his own life from reading some of the more imaginative fanfictions.
Tony would likely laugh at some of the more out - there plot ideas. He'd probably think he could come up with better technology than what's in the fanfic. Steve would be more open - minded and might enjoy the creativity. Thor would be like 'This is not like any Midgardian story I've heard!'.
Some Avengers might be amused. For instance, Tony Stark would likely find it a fun distraction from his usual high - tech world and might even make sarcastic comments about the plot or characters. He could say something like 'These wizards should really upgrade their transportation - brooms? Seriously?'.
Draco Malfoy would probably be outraged at first if he read fanfiction that showed him in a positive light towards Harry and his friends. He's so used to being on the opposite side, but then he might secretly be a bit flattered by some of the more complex characterizations of him.