You should strive to be in a positive and friendly attitude from the moment to meet your first person at the interview venue. Even if you are to later to face a really tough question that throws you, maintain this same positive and friendly attitude.
You are ultimately going to want a positive response from the interviewers by way of being extended a job offer. You cannot expect them to be positively disposed towards you if you do not maintain a positive approach yourself. A winning attitude will not fail to make a positive impression on the prospective employer.
One of the cornerstones of a positive attitude is a calm emotional state. If you are nervous to such an extent that you can't think straight, this will only undermine your confidence, performance and ultimately your chances. Your preparation and research should go some way towards calming your nerves.
When you're on the prospective employer's terrain, try and envisage yourself working there. That should help you calm your nerves, which are quite naturally present under the circumstances. A little nervousness is often a good thing because it will keep you on your toes. It will heighten your senses and clear your mind somewhat.