As a recruiter, we frequently get feedback that our extremely qualified, professional candidate has been ruled out of the running for the job because they “didn’t seem to want it.”  This response is ironic, as most people wouldn’t take the time to interview for a position they didn’t actually want.  So what causes this? 
Your body language can often times send a stronger message than what you are actually saying.  If you aren’t sitting up straight, seeming to be engrossed in the interview, you will come off as being bored and indifferent to the job opportunity.  No hiring manager will want to hire someone who doesn’t truly want to be a part of their organization for fear of losing their investment in about a year when another opportunity comes along. 
The article “Interview Body Language Mistakes That Can Cost You The Job,” in Forbes.com, gives body language tips to help you avoid sending the wrong message to the interviewer.