He probably feels angry at first. Harry has always been a bit of a rebel, and being forced to do such a task by Snape, who he doesn't particularly like, would surely make him fume. He might feel it's unfair, especially if he hasn't really done anything wrong.
Deep down, he could be a bit confused. He may wonder why Snape is making him do this. Is there a hidden motive? Or is it just another one of Snape's ways to torment him? This confusion could lead to him being more observant as he cleans, trying to figure out what Snape is really up to.
Harry is likely to be angry at first. He has always had a tense relationship with Snape, and this act of spanking would seem unjust and humiliating to him.
One possible reason could be punishment. Snape has always had a rather thorny relationship with Harry in the original series. He might see this as a way to assert his authority over Harry. Maybe Harry has done something in the context of the fanfiction, like accidentally messing up Snape's potion ingredients or getting into an area of Snape's house where he wasn't supposed to be.
Well, Harry is a brave character. So, he would probably be full of anger and disbelief. He might start yelling at Snape, demanding an explanation for such an inappropriate act. He would also be thinking about how to escape and report this to his friends or the proper authorities in the wizarding world, like Dumbledore.
Harry could react with anger. He is a strong - willed character. So, he might confront Snape about his hatred. He would demand to know why Snape is treating him this way when they should have a father - son relationship. This could lead to a series of intense arguments between them.
Harry might struggle and try to break free. He is a brave character and doesn't like being restrained, so he would likely use his physical strength and magic skills, if possible, to get out of Snape's hold.
Harry's reaction can be complex. At first, he may show immediate resistance, arguing that he's been wrongly accused. When he realizes that arguing won't get him anywhere, he might become sullen and withdrawn. However, this sullenness is also a sign of his determination not to let them break his spirit. He'll keep thinking of ways to prove his innocence or to show that their punishment is unjust. In some fanfictions, Harry also uses these punishments as a motivation to become better at magic, so that he can one day stand up to Snape and Draco more effectively.
On the other hand, he could feel ashamed. If he realizes that he has truly made a mistake, seeing his father so angry and disappointed could make him feel bad about himself. He might lower his head and not say anything, with a look of guilt on his face.
Perhaps Harry just stands there, silently crying at first. He might be in shock that Snape has been so cruel. Then, he could start to think about what Snape has said or done, trying to understand if there is any truth in it. Harry is a deep thinker, and this situation might force him to re - evaluate his relationship with Snape and his own actions in Hogwarts.
In some fanfictions, Snape might see this as an opportunity for his own growth. He might accept the apology gracefully, realizing that holding onto grudges isn't helping him. This could lead to a new relationship between them, where they start to understand and respect each other, or at least tolerate each other's presence.
Snape might initially be taken aback. He's not used to students standing up to him so forcefully. He could glare at Hermione with his cold, dark eyes, trying to intimidate her into silence.