An interview is essentially a communication with the representative of the company that wants to hire you. So you stand a chance to be hired only if the other person takes it forward.
Hence, it is just a game of influencing the interviewer to pursue your case for further consideration. The question now is, how would one influence the interviewer?
Not all interviwers are the same. There are also personal biases that comes into play. When I interview someone and if I do not like the way he is dressed, then I will unconsciously start noticing atleast some of the negative aspects. And sometimes these negative aspects weights more than the positive ones, how much ever small these negative aspects are.
So coming to the confidence part, you should not depict more confidence than the interviewer. If you do so, then you will be perceived as a competition for him. You must first guage how confidet the person across the desk is. Then accordingly tune yourself to his wavelength.
NLP can help you achieve this without any effort. It will help you guage the person inside out, and you will be at an advantageous position to exploit it.
Enjoy learning! Enjoy getting interviewed!