Great advice @arbitrarykitten! It's interesting to hear how other people do this.

I tend to start with motive and build outwards (not deliberately, that's just the way it tends to happen). What is the character doing and why --> what sort of character would find themselves in this situation --> what are the characteristics they would have / would need for this all to make sense --> flesh out details based on conversations.

I like your eye for the detail though. And particularly the secret, that's a really nice idea.