There is no doubt that job interviews can be extremely stressful. However, what can either increase or decrease your stress can be the way in which you react to the questions that are being asked during the interview.
An easy mistake to make in an interview is trying to figure out what the potential employer wants to hear. While they do in fact want to hear what you have to say, they also want the thought to be coming form you and they want your reactions to represent how you really are.
When you are yourself in a job interview it shows. Answers that come from the heart are ones that come off in a more natural way and therefore allow for a more confident and relaxed mannerism to flow forth.
On the opposite end of the spectrum, answers that are not coming form the heart, show as well. These answers are ones that come off as disingenuous and tend to give the impression of dishonesty. They are often preceded with the classic ummm or ahhh, which is a sure sign of non-confidence.
So, it is in your best interest to simply be yourself while being interviewed. Yes, you do want to sell yourself, but do it in a natural way. Attempting to sell yourself with a bunch of hot air is not the best way to start off a business relationship.
Besides, interviewers have seen it all. They seem to know when someone is telling them truth and they know when someone is telling them what they want to hear. In the end though, they not only want to hear the real you, but need to in order to determine if you will be a good fit for the position you are interviewing for. This cal can only come when you are being honest and being yourself.