Emotions are synonymous with and derive from our thoughts. Stanislavski said, "In every physical act there is a psychological element, and a physical one in every psychological act". Meaning that our thoughts lead us to actions. And if these actions are coming out of our investment of the given circumstances, they will lead to emotions. True-seeming emotions.
To understand the complicated thoughts and feelings of our character, we often times turn to examples of it in those we know. To truly see others, actors must possess another important quality...empathy.