In a fanfic, their first kiss might occur during a more emotional moment. For example, Beckett could be sharing a deep, personal story from her past. Castle listens intently, and when she finishes, he just leans in. It's a kiss that's both a comfort and a new beginning, a way to show that he's there for her not just as a partner in crime - solving, but also as someone who cares deeply about her on an emotional level.
The plot could be centered around a misunderstanding. Beckett might think Castle is interested in someone else, and this causes some tension between them. But as they work through the case, they start to realize their true feelings for each other. The first kiss could come when Castle finally clears up the misunderstanding, and they share a tender moment.
One interesting 'castle first kiss fanfic' story could be about two characters who are initially enemies in the castle setting. They constantly bicker and compete, but during a dangerous situation in the castle, like a siege or a mysterious threat. In the heat of the moment, as they are fighting side by side to protect the castle, they share their first kiss. It's a moment of realization that their feelings are more than just rivalry.
First, create vivid descriptions of the castle. Mention the cold stone walls, the echoing halls, and the mysterious rooms. This sets the mood. Then, develop your characters well. Decide what their personalities are like and how they interact in the castle setting. For the first kiss, build up the tension. Maybe they've had some close calls or misunderstandings before. And finally, make the kiss moment special. Use detailed descriptions of their emotions, like the butterflies in their stomachs or the quickening of their heartbeats.
In many Castle and Beckett first kiss fanfictions, the moment is often filled with pent - up emotions. Writers usually build up to it by showing the growing relationship between Castle and Beckett. It could be after a dangerous case, when they realize how much they mean to each other. The first kiss might be hesitant at first, like a gentle exploration of their newfound feelings.
Yes, it might be a plot point in some fanfic. Fanfic often takes existing characters and creates various scenarios, so it's quite possible that in a particular Castle - related fanfic, Castle could witness Vaughn kissing Kate.
One common element is the underlying tension. Before the kiss, there's usually a lot of unspoken feelings between Castle and Beckett. Another is the role of the cases they solve together. It often brings them closer and sets the stage for the kiss. Also, the use of dialogue is important. They might exchange some meaningful words right before the kiss that express their true feelings.
Your best bet would be to search on large fanfic platforms. Archive of Our Own (AO3) is a great place to start. Just go to the website, type in 'Castle' in the search bar, and then add keywords like 'Vaughn', 'Beckett', and'seeing a kiss'. Another option is Fanfiction.net. Navigate to the 'Castle' section and use the same kind of keyword search. There may also be some smaller, more specialized fanfic sites related to the show, but AO3 and Fanfiction.net usually have a wide range of content.
Another common theme is the change in the relationship. After the first kiss, the relationship between the characters usually shifts to a more intimate or committed level. It could be that they start seeing each other in a new light, like in a 'first kiss fanfic' where two colleagues who were just friends become romantically involved after that one kiss.