"Major characters emerge; minor ones may be photographed."-Graham Greene
I think this is an important point. I also think this is something we all struggle with. We try to get everything about a major character out right away and the result is the beginning of our manuscript becomes summary rather than scene. Too much of a rush. The major characters should be revealed little by little, as fits with the story, and they should slowly emerge into complex rounded characters. This is where SHOW instead of TELL does apply.
Minor characters are often snapshots. They may not evolve except in a way that promotes the advancement of story. They might just be a quick sketch that is useful to develop main characters or story.
Beware the manuscript that has no minor characters. If all you have are major characters then most likely something is wrong. I’m a big believer in equality but not in fiction. If all your characters are equal then something is probably wrong. The reader needs main characters to focus on and identify with.Character is everything. If a reader identifies with a character she will
excuse a lot. But one aspect of this is understanding that a major character
will emerge through the engagement of that character with the story.