Characters are shaped by their behavior. Lived experience. Hard choices. Hard knocks.
Characters make choices because of who they are. Likes, dislikes, priorities, weaknesses.
Who they are --> what they do.
What they do --> who they are.
A character with a personality and no lived experience is just an empty envelope.
A character with a really interesting life story whose personality only reflects the person they are now, with all the previous stages rendered irrelevant or inconsequential, is just a cardboard cutout.
This brief & cryptic commentary courtesy of my starting-a-new-book-brain.