Characters are pliable when they first crawl out of my head. Their traits, conscious motives and hidden drives can be twisted to fit the needs of the plot for a short time, but they begin to set as words are layed on paper, and soon the plot has to change to fit the exigencies of the new characters. This isn't necessarily bad. A plot that proceeds from what the characters naturally want to do is stronger, more credible.
two characters talking about women
"They should be deeper than us, ya' know? I mean, I always thought they were thinking about character and relationships and deep shit like that."
"Like philosophy? Life and Death and the Meaning of the Universe? Who ever got one into bed talking about stuff like that? They start looking over your shoulder to see who might be looking at them. You know what gets them into bed? Talking about how good they look or how much you love them." JIm hesitated, thought for a second, and gave us his final word on the subject. "You want to know what's important to a guy, you look at what he spends his money on, and it's the same with girls, so look what they spend their money on -- clothes and jewelry and cosmetics. Deep? Hell, you find a woman thinking about something deeper than her pores, you know she's got her mind on cellulite."