This kind of content is not appropriate and goes against common decency. We should focus on positive and meaningful subjects instead.
Reactions could vary greatly. Some might freeze in panic, while others might try to explain or make excuses. But in any case, it's a highly stressful and embarrassing moment.
One possible reaction of Sirius could be shock. He might stand there frozen for a moment, unable to believe what he's seeing. Another reaction could be anger. If Sirius has certain expectations or feelings about James and Regulus' relationship, he could get angry at finding them together. Or he could be confused, trying to figure out how and why this is happening between his friend James and his brother Regulus.
Well, Ron could get angry. He has a strong loyalty to Harry, and seeing Harry with Draco, who he has always seen as an enemy, might make him feel betrayed. He might immediately jump to conclusions and start shouting or demanding an explanation from Harry.
She might be a bit shocked at first. But then, being Sally, she'd probably quickly regain her composure and give them a knowing smile. After all, she's seen Percy grow up and she would be happy for him to have such a close relationship with Annabeth.
Well, depending on the context of the fanfiction, Sirius could also be very understanding. If he knew that Harry and Hermione were just having a private conversation about important matters like their fight against Voldemort or schoolwork, he might just give them a knowing smile and say something like 'Carry on, then. Just making sure everything's all right.' He's a rather perceptive character, after all. And he trusts Harry a great deal, so he would likely give them the benefit of the doubt before assuming anything untoward.
One possible reaction could be shock. Carlisle might be momentarily stunned to see them in an unexpected situation. He could quickly compose himself and offer some words of wisdom or caution. For example, he might remind Edward to be extra careful with Bella considering her human nature.
Another possibility is that Lucius could be initially confused. He might not understand what he's seeing at first. After all, Harry and Draco are supposed to be enemies in the general wizarding world view. He could stand there for a moment, trying to process what's going on before taking any action.
Sirius may also be angry at first. Given his protective nature towards Harry, if he misinterprets the situation as Draco causing trouble for Harry, he would likely step in assertively. But if it's a situation where Harry and Draco have a more positive or friendly interaction in the fanfiction, Sirius would have to re - evaluate his initial reaction.
There could be various reasons. Maybe she feels neglected in her relationship with dad. If dad is always busy at work and doesn't give her enough attention, she might be drawn to someone else who seems to care more. Another reason could be that she has some unmet emotional needs. For example, if she wants more excitement in her life and dad is a bit too routine - centered.
One possible reaction could be shock from the other students. They know Snape as a strict and often unfriendly professor, and Harry calling him 'dad' would be completely unexpected.