I have, on a number of occasions been on job interviews that, during the interview, I decided I did not want the job. I then proceeded to shoot myself in the foot and lose the interview, sometimes regretting it. The Interview is NEVER the time or place to decide whether or not you want the job. Always sell yourself throughout the process. Only when you have a firm offer in front of you is the time to decide whether or not you want the job. Sometimes after thinking about it you may decide to give it a shot!
DonD