Its important to maintain a competitive posture in the interview. The employer should be aware that they are not your only suitor. There is a delicate balance between letting the employer know that you really want to work for them and that if they don't make an offer, you will go with another company. The best way I can illustrate it is with comparing it to dating. Sure, you love him/her and only him/her, but if things don't work out, there are plenty of other hims/hers banging on your door asking for a date. Right? Well, maybe it doesn't equate directly to your personal life, but you get the general idea.
The perceived value of any product is directly proportional to the number of people fighting for it.
This posturing is very simple to incorporate into your interview language. Frame it in the form of a simple 1-2-3 engage/disengage/re-engage statement. Example:
If you feel comfortable with closing the sale, you can add the "Are you ready to make an offer?" question to the last statement above. The point is that you have put a limited time offer on your enthusiasm—if they want you, all of you, they better move quickly and decisively.