See if you can get a position description before you go in for the interview. Be certain, however, to avoid giving the impression that you think you know all about the position or the organization. Part of the interviewer's job is to provide you with information and your giving the appearance of knowing everything already might be offensive or irritating. It is natural to be nervous in anticipation of an interview. Remember, though, it is a meeting of those with a mutual interest.
Interviewers are sometimes stern without intending to be.
You are there at their invitation. A pleasant, engaging manner on your part will make it easier for the interviewer. Maintain eye contact and be certain that your body language is natural and complementary to the discussion. Be prepared to talk about yourself. Some interviewers will give you a chance to make an opening statement.