You must create the character's internal life. What do I mean by internal life? I mean the thoughts, feelings, memories, and inner decisions that may not be spoken. When we look into the eyes of actors giving fully realized performances, we can see them thinking. We're interested in what they're experiencing that may never be spoken, that quality of nonverbal expression - which is as much a part of the characters as breathing and as real as what they say and do. This is their internal life. It helps us believe in the characters and care about them.