They might be characters from a specific genre of story. For instance, if it's a romantic comedy, Sonny could be a bubbly girl and Chad a bit of a shy but charming guy. They could be neighbors or coworkers, and their relationship develops in a humorous and heartwarming way.
Power is a main theme. The Corleone family's rise to power, their struggles to maintain it, and how power affects their relationships both within the family and with the outside world.
Sonny might become more confident in her relationships. The kiss could make her realize that she can be bold in matters of the heart. Chad, on the other hand, might become more vulnerable. He has always been a bit of a ladies' man, but this kiss could change his approach to relationships.
Sonny might be a werewolf pup who has a problem with Mommy, who could be Leah as a mother - like figure in the werewolf pack. The 'shut up' comment could stem from Sonny's disagreement with Mommy's decisions regarding pack matters, like how to deal with the vampires in their territory.
One plot idea could be that after the kiss, they struggle to define their relationship in the context of their showbiz world. They might face pressure from the studio or fans.