Some books have characters which are so richly developed that, before the ride’s over, you feel intimately connected with them. You can picture them practically standing in front of you. You know their likes and dislikes, their mannerisms, even what they would and would not say.
That has always fascinated and amazed me; that I could say, in my mind, “No! Juan would never do that! How could his friends think he would?” The author did a great job developing that character.
I’ve found that I only come close to that when I take enough time to fully think the character through in my head. Sounds obvious, I’ll admit, but sometimes you just want to delve into the story and you don’t give your character a chance to “breathe” and fully come into being. I suppose you could say characters need time to grow into their own, and we need to be patient enough to let it happen.