Sirius would be in a state of panic at first. But then he would quickly gather his wits. He'd likely try to find out what caused the seizure, whether it was a spell, a potion, or something else entirely. He might also call for Remus or other trusted friends for assistance, as he wouldn't want to leave Harry alone in such a vulnerable state.
Sirius would probably be very worried and immediately try to help Harry. He might use some basic healing spells he knows.
In this fanfiction, the seizure could be part of a curse that was meant for Sirius but backfired on Harry. Harry starts seizing in front of Sirius, and Sirius is filled with guilt and determination to reverse the curse. He uses his knowledge of the wizarding world and his connections to search for a cure, all while taking care of Harry and keeping him safe from any potential threats.
Snape might initially be suspicious. He has always had a distrust of Harry, so he could think it's some sort of trick. But then, as a Potions master and a teacher, he might also feel a sense of responsibility and try to help.
Sirius would probably be overjoyed at first. Harry is his godson and he cares deeply for him.
Harry would likely be quite curious. He'd want to know what these fanfictions were about. Maybe he'd be a bit cautious at the same time, worried that some of the fanfictions might distort Sirius's true character. But overall, I think he'd be intrigued and might even try to find a way to read some of them, just to see what the fuss was all about.
Ginny might be extremely embarrassed. She would probably blush deeply and try to hide behind Harry at first.
As time goes on, Harry could start to explore what it means to 'please' Sirius. He might recall their past relationship, the things Sirius likes and dislikes, and try to use that knowledge to his advantage. He may also feel a sense of loyalty towards Sirius that, despite the odd circumstances, makes him more accepting of the situation. For example, he might remember Sirius' love for adventure and try to engage in some magical adventures with him in a way that 'pleases' him.
Sirius might immediately pull Harry into a tight hug. He would be very protective and try to comfort him. He might say some soothing words like 'It's all right, Harry. I'm here for you.'
Well, their relationship would be in tatters. Harry would probably avoid Sirius like the plague after such an event. Sirius, on the other hand, would be at a loss for how to deal with what has happened. He might feel betrayed, even though he knows Harry was forced. This would create a huge rift between them that would take a lot of time and effort, if ever, to mend. It would also change the way they interact with other characters in the fanfiction, as their relationship was an important part of the overall story dynamic.
Harry may also feel a sense of adventure. Despite the situation, he's used to facing strange and dangerous things. He might start to think about how he can use this situation to his advantage, like finding out more about his parents or the secrets of the wizarding world that Sirius might know. He could be a bit excited as well as scared. For example, he might be thinking that this could be a chance to get away from the Dursleys for good.
Harry might be initially shocked. He could feel a mix of sadness and a sense of responsibility. After all, Sirius was like a father figure to him.