Whether it is a candid moment at the water cooler in between managers, or calling to follow up after an interview. You want to be polite and courteous to all parties related to the prospective employer. Be aware of your body language. Smile and laugh when appropriate. People who smile are seen as individuals who are human, more accessible and open.
Confidence is conveyed by an upright posture.
Stand up straight when you walk, and sit up when you are facing them behind the desk or conference table. One does not slouch at an interview. A firm handshake is another measure of confidence. Do not speak in a monotone. You want to vary the tone and volume of your voice during the interview.