The plot could develop in many ways. For example, if it's a mystery - focused fanfic, Beckett's presence at the hotel could be linked to a big mystery. Castle, being a mystery writer, would be eager to get involved. They might start investigating together, exploring different parts of the hotel. As they do so, they could face various challenges and obstacles, which would bring them closer together. Along the way, they could also discover secrets about each other that they didn't know before.
First, Castle's emotions would play a big role. If he's shocked but excited, he might immediately start flirting with Beckett. But Beckett, being professional, could try to keep things on a work - related level. However, as they start to dig deeper into why Beckett is at the hotel, they could uncover a conspiracy that involves powerful people. This would force them to rely on each other more. They might have to go undercover within the hotel, pretend to be a couple perhaps. As they do this, their relationship would evolve, and they could end up not only solving the mystery but also finding a new level of connection, whether it's romantic or just a very deep friendship.
He might get jealous right away. He could storm off and then later come back to confront Beckett about it.
One possibility is that Castle has booked the hotel for a romantic getaway by himself, and seeing Beckett there completely throws him off. Beckett could be there for a much more serious reason, like following a lead on a dangerous criminal. Castle may start to worry about Beckett's safety and decides to stay close to her. As they interact more within the hotel, they might realize how much they've missed each other's company, and this chance meeting could be the start of something new between them, whether it's a new level of friendship or a re - awakening of their romantic feelings.
He could react with anger. After all, if he has feelings for Beckett, seeing her with someone else like Vaughn might make him furious. He might lash out at both Beckett and Vaughn without really understanding the situation.
He might react with jealousy. He could glare at Josh and then give Beckett a hurt look before storming off.
One possible plot could be that Castle sees Beckett with Josh and it makes him realize his true feelings for her. He might then start to compete for her attention in a humorous and endearing way, like writing a new novel inspired by his jealousy and love for her.
Perhaps the man is a new partner Beckett has been assigned to work with on a case. Castle sees them together and his insecurities get the best of him. He starts imagining all sorts of things, like Beckett replacing him in her life. However, in the end, it turns out that Beckett has no romantic feelings for the other man and Castle realizes he overreacted.
In a fanfic where they have a baby, their relationship would shift. They would now be partners not only in crime - solving but also in parenting. Castle would likely become more domesticated, helping with diaper changes and midnight feedings. Beckett would have to learn to let Castle in more on the parenting front, and they would communicate more about family - related matters. This new aspect of their relationship would add a different layer of complexity and closeness to their bond.
Castle might become more responsible. He was always a bit of a carefree writer, but with a baby, he has to grow up a bit. Beckett, on the other hand, could become more empathetic. She was tough as a detective, but motherhood softens her edges.
Another thing that could occur is that Castle decides to confront Beckett about what he saw. He could be really straightforward and ask her what's going on between her and Vaughn. This could lead to an argument or a really honest conversation about their relationship.
It could be about Beckett having a series of strange encounters at the hotel. Maybe he meets an old acquaintance who brings back memories, good or bad. For example, an old friend who he had a falling - out with and now has to deal with the unresolved issues in this new hotel setting.