It might have been the introduction of The Speaker for the Dead where Orson Scott Card talked about how each character fits into a family and their relationships within.
One of the characters had six children, and he said each fit into the family in a unique way. It was hard for him because every time two people get together, whether it be in a story or in real life, their character is a little different.
It’s these unique relationships that stretch writers as we try to show the stories that are in our heads. As he explained it, each of those six children and the mother had different facets depending on which other characters s/he interacted with. It comes up to a lot of facets, then slightly more shadings as you combine more than just two characters in a scene.
As I write, I try to listen to the character as I’ve imagined him/her. It helps, but sometimes it isn’t enough to preserve those slight differences with each character. As with everything else, practice makes perfect.