I was surprised to discover that they're making another Percy Jackson movie. I saw the Lightning Thief and thought it was an abject failure. I had high hopes and they were dashed.
The gold standard of screen adaptations is John Huston's work on The Maltese Falcon. If you read the novel by Dashiell Hammet, you might notice that the fat-man, Gutman, has a daughter. She doesn't contribute much to the story and thus the scene where Sam Spade talks to the daughter can be safely cut from the screenplay.
Screen adaptations are a game of Jenga where you pull out pieces of the story. Some, like Gutman's daughter, can be removed with anyone noticing. Others leave gaping holes. Or removing some story elements can cause the narrative structure to sag or to collapse altogether.
The art is in picking what to cut.
Some story elements can't be removed, but can be combined. Let's suppose Percy Jackson has two female classmates, suppose further they are both daughters of Greek gods of war: Ares, and Athena. Perhaps they can be combined. That would be a good idea if the girls both had the same sort of relationship with Percy, but in the novel one is an antagonist and the other an ally. The movie combined the two and the result is a high-maintenance girlfriend. Sorry, didn't work for me.
Other story elements can't be changed or the character of the movie changes drastically from the book. In the Lightning Thief novel, the school for god-kids is a big old mansion surrounded by Elysian fields and hills. It has some cottages but I got a definite Woodstock Yasgur's Farm/Strawberry Fields forever vibe from the prose. The movie moved the camp to woods that was recycled from some slasher pic. The atmospherics changed completely and not for the better. Every screen adaptation will have to deal with fan boys like me who are married to a mental picture they've formed of what Middle Earth or Hogwarts looks like.
Underneath all this is the fact that stories are told through distinct media: prose, ballad, radio drama, or film. Each medium brings with it constraints and advantages.
The writer knows the story, but must also be aware of the medium able to work through it. The magic is made by telling the story in a way that leverages the advantages.