It begins even before you say your first word in an interview. As the interviewer walks toward you to shake hands, an opinion is already being formed. And as you sit waiting to spew out your answers to questions you've prepared for, you are already being judged by your appearance, posture, smile or your nervous look.
Look back at speakers or teachers you've listened to. Which ones stand out as memorable? The ones who were more animated and entertaining, or the ones who just gave out information? This is not to say you have to entertain the interviewer -- no jokes required -- but it does mean the conversation should be animated and interactive. If you say you are excited about the prospect of working for this company but don't show any enthusiasm, your message will probably fall flat. So smile, gesture once in a while, show some energy and breathe life into the interview experience.
And don't underestimate the value of a smile. In addition to the enthusiasm it expresses to the interviewer, smiling often makes you feel better about yourself.
Preparing what you have to say is important, but practicing how you will say it is imperative. The nonverbal message can speak louder than the verbal message you're sending.