Interviewing prospective employees is an amazing process. How long does an interview last? Ten minutes? Thirty minutes? Maybe an hour and a half if the interview is over lunch? And yet, even over such a short amount of time, it is amazing how some interviewees cannot keep from revealing their true natures. They are doing something that will likely change their live in a significant way, and they can't put on a good show for even that small amount of time.
I don't mean to imply that someone should put on a false front, but interviewing is like a first date; the other person knows you possess some flaws, but they want to feel like you respect them enough to forgo slurping your soup just this once.
So, make sure your phone is off before you walk into the interview. Unless your wife is nine months pregnant, don't even check who is calling. Be super nice to the support staff, because they may well be asked about you. Don't be late, and don't act rushed.
For a list of ways to blow an interview, go to Top Ten Ways to Blow a Job Interview.