I agree with what both PA and Eric said.
There is also another side to the story that people often do not realize. The interviewer is undergoing the same type of stress at an interview as you are. Some thoughts that go through his mind before the interview?
1, I read his application. I know she is well qualified for the job. What if I say something wrong and she changes her mind about working for us?
2. Her qualifications are so good, I am afraid the salary I can offer will be an insult to her.
3. What can I do to show her that we really want her to come work for us, without it looking condescending?
When I was interviewing applicants I used to experience very much the same things you are. The interview is stressful to both the interviewer and the applicant.
Keep in mind your application has already impressed the interviewer. The interviewer sincerely wants to hire you, but he can not appear to be too eager.
Every interviewer looks upon every applicant as potentially becoming his boss someday, he is trying to make a good first impression, not just evaluating you. Remember, the interviewer sees you as interviewing him.
Do your best to put the interviewer at ease. Remember you are in control of the situation. You can walk out the door at any point in the interview. He is stuck there and can't leave. Try to see the interviewer as a potential friend. Do your best to keep from intimidating him and begin with a desire to see the best attributes of the interviewer.
Remember, you are the one hiring the interviewer to serve as your guide through the jungle of the company. He is working for you. Treat him as your valued servant and reward him with appropriate words of kindness and justified thanks.