Believability is the key to empathy, and empathy is the key to success.
You are believable when you appear to be in your character's situation - when you seem to be inventing the music and words as spontaneous responses to what the character is experiencing. This requires that you understand every word that you sing and that is sung to you and that you make the connections between your thoughts, your feelings, your breathing, and your body that we do in real life.
If you are struggling to get into a character or if you prefer to use technical acting, be sure to incorporate these behaviors: