And the other characters need to show off their flaws, too.
An ensemble cast means I need to differentiate the characters from each other, otherwise, they end up being redundant. And I can't just let them grow spontaneously because they tend to grow more similar if I do that.
So, I consider what flaws they could have to contrast with each other. Also, what virtues. What interests. Whatever else comes to mind to be orchestrated.
sigh. The muse does like complicating my job.