I was helping out in an interview process recently and was surprised by something. There was a group of us interviewing candidates, and we had a set list of questions. Each of us asked a question or two and all of us took notes to discuss afterwards. However at the end of each interview, the coordinating interviewer asked each person this question:
If you were offered the job, would you accept?
During one of the discussions, I noted that I thought this was a waste of a question. Certainly everyone would answer "yes" immediately. After all, I have when I've been asked the question.
However the coordinator said that he'd asked this question many times and learned a few things about people. They may hesitate, they may invoke conditions, they may not say yes.
Needless to say I was stunned until I read something similar in the Ask the Headhunter newsletter. One of the questions he answered was on telling the interviewer you want the job, which is very similar.
You should learn to say "I'm interested in the job" if you at all are at the end of the interview. This doesn't bind you, and circumstances may change. Perhaps the offer will be low, perhaps you'll get another offer. You don't know, but at the time, express interest. If someone asks you if you still want the job after the interview, say yes unless you are sure you do not.
There are times you don't want the job, but otherwise, just learn to say "yes, I want the job."