I understand, you might learn about their history and their personality from bios but can't really see how the react with others or how to handle certain situations.
And sometimes it's hard to put it into words sometimes and it doesn't match how the character actually is... I've thought about my character's development but I'm keeping it to myself until I get the story thought out better.
That's one of the most fun part to do I think. It's cool to see how characters matured over time, in personality as well as abilities, and how their views changed for better or worse... I agree, a lot of people forget about it even though it's the most important...
Honestly being a writer is the best when in comes to character's stories, although the get kind of unnoticed in the naruto fandom.