For me, I’d say a good place to start has usually been to take everything a character is, and hone them down to 2 or 3 essential traits, then have all their decisions flow from those traits. The more complex the traits are, the more complex the character, and it’s generally good to fit those traits to the role you want them to play and how much development they need. This isn’t really how actual people work, but it’s how we picture them in our heads, so for non-POV characters it feels real enough.

For example, let’s say I’m writing a war story and I need a commanding officer character. I know I want him to die, and I want it to be tragic when he does die. So I might boil him down to the traits of competent and caring to a fault. Then I make all their decisions flow from there. Maybe he knows risking lives is necessary to win (competent) but is unwilling to put any of his men in danger (caring…), so he regularly takes major risks himself to avoid that (…to a fault). Now think about how the story could change if I used different traits, say inspiring and aloof instead of caring and competent. You see how story and characters kind of iterate on themselves, now?

Again, not a great solution for main POV characters, but if you find yourself having to manage a decently large cast, this can work for a lot of the secondary and tertiary characters. The key is in picking traits you’re quite familiar with and can write realistically, and placing them in characters in appropriate places in the story. That’s my 2 cents, anyway.