This rule applies even if you know you may dress casually when actually employed. Pay particular attention to personal grooming details such as hair, nails and teeth. Make sure your clothes have been cleaned and pressed. Shoes should be in good condition and shined. People in America often look at a someone's shoes to get an impression of the person.
First impressions count immensely in an interview situation.
Greet the interviewer warmly and make eye contact. Shake hands firmly when you meet the interviewer. Be yourself at the interview and try to communicate directly to the other person's needs. Try to take the other's person's point of view. He or she needs a job done.