B1C9

Writing conventions

The first writing convention I found fascinating was that protagonists of young adult novels are nearly always orphans. This is because the protagonists need to get into a lot of danger for the story to be any good, and any protagonist with parents would simply not be allowed to get into said danger. If a young wizard with parents tried to go fight a murderous evildoer, his parents would say, "I don't think so; we're calling the police." Thus, the parents have to die.

Here are some other conventions I've noticed:

In romance novels, the man needs to be very rich (so that he is attractive), fairly young (so that he is attractive), and have a lot of free time (so that he can woo the woman). The usual solution is that the man is an heir to a family business which makes him rich. Unfortunately, this also makes him unattractive, because self-made men are more attractive than nepo babies. Thus, the story spends time establishing that even though the man is an heir, he is actually somewhat self-made because he paid for college himself, or created a new division of the business.
In classic murder mysteries, the list of suspects needs to be fairly defined. The story is no fun if a stranger the readers have never met ends up being the murderer. Thus, the story spends time establishing that the room/castle/island was inaccessible to strangers, and only a limited cast of suspects could have done the deed.
In (auto)biographies, readers demand some sort of a narrative even though real lives are random, and definitely do not have a clear story arc with an inciting incident, rising action, climax, and falling action. The easiest way to force lives to fit this pattern is to focus on the subject's childhood as foreshadowing for what would happen in adulthood. I think this is almost always bullshit because the people who remember the subject's childhood (the subject himself, his parents, and his close friends) are very biased, and it is very easy for them to cherry-pick only the incidents that add to the story arc.
In fantasy novels, the main character has to be both unskilled (for maximum relatability) and powerful (to beat the baddies). Thus, the story usually has the hero find artifacts left behind by an extremely powerful culture/wizard/deity. To explain why no one else could operate these artifacts, the story makes up something about the artifacts only being accessible to the Chosen One. (From ACX.)
In fantasy novels, the main character needs to spend time gaining skills and allies. Meanwhile, the author wants to flex their world-building skills. The synthesis of these desires is that most fantasy books spend 80% of their plot either on a road trip or in a school, so that the main character has a natural reason to run across the various world-building details the author has created.
In television shows and movies, the characters almost never spend long periods of time watching in-universe television shows and movies that the viewer also watches. Weirdly, this is not true for books; book characters can spend long periods of time reading an in-universe text along with the reader. I'm not sure why text is a more recursive medium than big-budget video. In contrast, low-budget video (e.g., YouTube, TikTok) is frequently recursive, as seen by the extremely popular "watch me react to someone else's video" genre.
Characters in contemporary stories have the minimum number of siblings required for the plot to work. For example, both Cinderella and Harry Potter needed to be mistreated by not-quite-siblings living with them. Cinderella, being an older character, got two stepsisters, but strictly speaking, she only needed one narratively and the other one was redundant to the plot and characterization. Harry Potter, being a contemporary character, only got the minimum required one mean Dudley. Related, George R. R. Martin wrote and then deleted a blog post where he was mad that screenwriters for House of the Dragon optimized the number of siblings in one of his fictional families (spoilers).
Main characters in stories have same-sex siblings way more frequently than people do in real life. Giving a main character a same-sex sibling is an easy way for writers to contrast the main character with someone who grows up in very similar circumstances and makes different choices.
The main cast of characters for most books have names beginning with different letters. This makes it easier for readers to follow along without confusing, for example, Adam with Alan.