Visual impressions are extremely important. The interviewer will make judgements about you based on your attire alone. When there is a question about casual dress versus professional dress, always dress professionally. Men should wear a dark suit, pressed shirt, clean tie that hangs to the belt and shined shoes. Women should also wear a suit, conservative blouse, hose, polished pumps and moderate makeup.
Body language is also extremely important.
Look the interviewer right in the eye. Maintain good posture while seated. Smile and laugh when appropriate. Focus on answering the questions completely and concisely and be sure to avoid rambling. Remember to pause after an important question.