Carlisle's usual calm and centeredness is challenged when he sees Bella's life in danger and his family divided over what to do with her baby. As much as it pains him, he agrees to do his best as a doctor to help Bella deliver the baby according to her wishes. He throws himself into research to figure out the best protection for her and for the fetus.
Jacob and Seth often remark on Carlisle's humanness and goodness. The strength of Carlisle's conscience comes to the forefront when he doesn't allow Jacob to fight his own pack to protect the Cullens and when he refuses to ask his friends to stand with them in a fight against the Volturi.
For more info on Carlisle, check out his "Character Analyses" in the other books of the Twilight series on Shmoop.