When you interview, your gestures and body language are speaking louder (and truer) than your words.
First off, the way you go dressed. Make sure you are impeccable in every aspect. Make certain you enjoyed some mints before the interview and that your hands are free from any signs of nervousness (sweat).

You should feel confident and inspire trust. Do not cross your arms or legs: if you have nothing to hide… then why are you hiding? Crossed arms and legs are often interpreted as closing yourself for protection (why are you afraid?), for avoiding showing who you are behind that ‘wall’ (what are you hiding?), of intent to separate yourself from your interviewer (why are you rejecting me?), or a combination of the above.

If you can’t hold eye contact, you will be sending the signal that you are not being truthful or that you are too insecure (for the job). On the other extreme, if you keep staring fixated in the other person eyes all the time, the impression could be that you are trying to hypnotize your interviewer, or pierce them with your eyes: not good!

A smile is good, it shows you are relaxed and capable of good humor. If you smile all the time, it may appear as if you are a clown, completely insecure or an idiot. Not good.

So what to do? Be natural, relax, enjoy the encounter, provide the information requested, ask questions of your interest. Any attempt to conceal, force, pretend, represent what you are not, etc. will be perceived as untrustworthy and undeserving of the job. Those encapsulated and rehearsed messages may sound convincing by themselves, but in the context of all other verbal and non-verbal messages, they will be contrasting and inconsistent: death giveaways of deception.

Are you nervous? It is OK to be nervous during a job interview, is almost expected, so admit it, smile about it (once) and move on with the interview in a sincere, open and friendly manner. In the long run, this is the best way to get the job you want.