Here's an interesting article that summarizes a recent scientific study from the University of Guelph in Canada about interview anxiety. Most people get a little nervous before interviews. For us anxious folk, they can be particularly excruciating; I know they were for me in my dark days. One time I excused myself mid-interview to go freak out in the bathroom for 15 minutes, which included calling my husband and begging him to come pick me up. After the panic passed, I worked up my courage and even went back in. All they said was "thanks for coming, we'll call you...." I didn't get the job.
The fact is that people who are anxious entering an interview often do not get hired, which is just a super fun Catch-22. This recent study looked at what these anxious people were doing that so put off their interviewers. They looked at many different mannerisms and traits - things like adjusting clothing, fidgeting, averting one's gaze, or one's rate of speech.
However, the key finding of the study was that all of those nervous behaviors matter much less than an interviewee's assertiveness and personal warmth. In short, if you are anxious before an interview, don't worry about how you look or what you say, focus on being genuine and warm. It's okay to tell an interviewer that you are a bit nervous or to show that you are not a robot; it's okay to admit that you have a life outside of work. You also want to be assertive - be prepared to ask questions, sit across from your interviewer, believe that you are the right person for the job.