Needs of the plot should never dictate a character's action. I don’t like reading books where a character does something that feels out of character, just to advance the story or set up a situation. That’s why I don’t plan too in depth. Unless I’m writing a sequel. Then it’s not so bad because I already know the characters and how they’d react.
So that’s why I don’t like writing pages of character study either. The most I’ll know about a character before I start writing them is their age (roughly), their gender, and the immediate situation they find themselves in. I just like to let whoever they are reveal themselves as I go. That, coupled with a very flexible outline, means I can let the characters dictate the direction of the story. I think it feels more natural, because they’re the ones driving it forward.
I’m often surprised when a character does something or says something I wasn’t expecting them to. They take on a life of their own, reacting in ways I didn’t expect them to because I never planned for it, and I think the story and the characters feel more real as a result.