It was a shock to me when I started interviewing for jobs- and an American friend told me, its very important in the US, when interviewing to say "I'm really excited (more than you really are) about the opportunity to work at XYZ." The truth could be you hate the company or are ambivalent- but if you don't say you are absolutely excited- the assumption is you are not interested and therefore will be passed over for someone who is 'truly excited'.