The relationship would be completely upended. Harry would be so ashamed that he might not be able to look Sirius in the eye again. Sirius would struggle to come to terms with what has happened to Harry and might even blame himself for not being able to protect him. This could lead to a downward spiral in their relationship, with Harry withdrawing and Sirius trying desperately to fix things but not knowing how. It could also have a ripple effect on the relationships they have with other characters, like Hermione and Ron, who would be shocked and unsure how to help their friend.
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.
Well, at first, Sirius would be in a state of shock and panic. But then he'd have to step up. Their relationship would shift from one of equals (in a sense, as Harry was growing up) to a caretaker - cared - for dynamic. Sirius would have to learn about Harry's likes and dislikes as a baby. He'd sing lullabies to him, comfort him when he cries. This new experience would make their bond stronger in a very different way. Harry might not remember all of it when he's turned back, but there would be a subconscious change in how he feels towards Sirius, a deeper sense of security and love that stems from those baby - days when Sirius was his whole world.
It's possible that in the cave, they start to see each other as equals rather than just a mentor - student relationship. They might argue about the best way to deal with the cave's mysteries at first, but through that process, they learn to value each other's perspectives. Also, if one of them gets injured in the cave, the care and concern shown by the other would enhance their relationship.
Their relationship would be severely damaged. Harry would likely feel betrayed by Sirius. After all, Sirius is supposed to be one of his protectors. It would be a huge breach of trust. Sirius, on the other hand, would probably be filled with guilt if he realizes his mistake. He would have to work hard to regain Harry's trust, if that's even possible in the context of the fanfic.
It could potentially strain the relationship. If Sirius misinterprets the situation, it might create some tension. Harry and Hermione would feel like they need to explain themselves, and Sirius might be a bit standoffish until he gets the real story. For instance, if he thought they were hiding something from him, it could make things a bit rocky between them for a while.
Well, one possible reason could be a dark magic spell that forces Harry to do such a thing against his will. Maybe a powerful dark wizard has put a compulsion charm on him. Another reason could be a misinterpretation of a prophecy that makes someone believe Harry must perform this act to prevent a greater evil. However, this is a very strange and dark concept for a fanfiction.
The relationship could become strained. Sirius might start to view Harry differently, with a bit of wariness. He might not trust Harry as much as before in terms of making good decisions. But it could also lead to a more open relationship where they talk about things they previously avoided, like relationships and feelings.
It may explore the emotional bond between them. Sirius could be like a father figure to Harry. He gives Harry advice and guidance, which is different from what Harry gets from others. This special connection makes Harry trust Sirius more than anyone else.
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.
It could develop in a complex way. The spanking incident might initially create tension between them. Harry could be angry or resentful towards Sirius at first.
In this type of fanfiction, the relationship is often explored through emotional depth. For example, Sirius might be portrayed as a mentor to Harry, guiding him through the difficult world of magic. It could also show their shared experiences of loss, which bonds them closer. Maybe Sirius shares his memories of James with Harry, strengthening their connection.