I spend a lot of my editing energy helping my clients refine their characters. And in my own writing I can never know too much about my characters. But readers don’t need (or want) everything about the inner and outer lives of our characters presented like a stenographer’s notes. It’s my belief that character is revealed through action on the page, but the motivation for that behavior is all off-screen. And motivation is the pulse of a story. That’s what keeps us turning the page.
We’ve all seen the iceberg analogy, and it’s one of the most universally true writing axioms: 80-90% of the story is behind the scenes and, in my opinion mostly embedded in characterization. Characters, though, are their own icebergs.
Screenwriting guru Scott Myers explains here: “Screenwriting Tip: Character Work as Iceberg.” https://link.medium.com/9ufJeoRdRS