They are likely to be horrified. James, for example, would probably be extremely angry that his friend had to endure such a thing. He might immediately vow to protect Sirius from any further harm. Remus, being more empathetic, would be there to comfort Sirius and offer his support in a more quiet and understanding way. Peter, though not as strong - willed as the others, would still be shocked and follow the lead of James and Remus in trying to make Sirius feel better.
The marauders' reaction would be quite complex. They might feel a sense of nostalgia as they read about the world they once knew, but through a different lens in the fanfiction. They could also be a bit critical. If the fanfiction didn't seem to respect the essence of the original story, they might be quick to point it out. However, if it was well - written and added new depth to the story, they could end up really enjoying it and even discussing it at length among themselves.
Remus might be a bit more reserved. He'd approach the fanfics with caution, but he'd also be touched by the stories that explored his struggle with being a werewolf in more detail. He might find comfort in knowing that fans understood his inner turmoil to some extent.
They might be very amused. The marauders are known for their sense of humor and the 'A Very Potter Musical' fanfiction is often full of comical and over - the - top elements. So they'd probably have a good laugh at the silly situations and the creative liberties the fanfic takes.
In this fanfiction, perhaps James Sirius Potter could learn some of the classic pranks the Marauders were famous for. He might get into all sorts of adventures, following in their footsteps and causing a bit of chaos at Hogwarts just like they did.
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.
He is often very suspicious. He might immediately think it's some kind of trick by the Marauders or their descendants. He'll likely try to figure out how to use it to his advantage against those he dislikes.
The main character is James Sirius Potter. As for the Marauders, James Potter would be important as he's James Sirius Potter's father. Sirius Black would bring his wild and rebellious nature into the story. Remus Lupin with his kind heart and werewolf secret could add depth. And even Peter Pettigrew, despite his later actions, would be part of the group dynamic in the fanfiction. There might also be other characters from Hogwarts during the Marauders' time or James Sirius Potter's time who interact with them.