Tell them what the objective process is and let them know they are being treated like all of the other candidates. When someone walks out the door, no one should feel bad, it should promote good will.

The worst situation for both parties is to have an interviewer or candidate who is ill-prepared. The candidate can and usually does pick up on an interviewer that is not prepared. In fact, I tell my clients that they should go into an interview prepared to help guide a bad interviewer [just in case that person isn't prepared] by selling themselves to that organization.