A colleague and I were debating this question today: Near the start of an interview, if you pretty quickly realize that the candidate is not going to be selected for the position, what is the most courteous way to proceed?
We both agreed that we tend to do a "courtesy interview" at that point (meaning that since the candidate prepared and made time to talk with you, you proceed with the interview), but we differed on how long to spend on it -- I argued 30-40 minutes is polite, and my colleague argued for 20 minutes (assuming the candidate didn't come from out of town, in which case he'd spend more time).
I think 20 minutes feels rudely abrupt. On the other hand, there's a strong argument to be made that going beyond that is wasting the candidate's time (and our own) if you know you're not going to hire her.
In phone interviews, if I realize a candidate isn't quite right, I'll often tell them during the call itself, as an explanation for cutting it short. But that's when the reason is easily explainable -- we're looking for someone with more of a background in ___, we'd need you to start months before you're available, or whatever. By the time they get to the in-person interview, they've been through a phone screen and thus any reason that would be quickly noticeable would be of a different sort -- for instance, the position requires great people skills but you're mumbling and can't make eye contact, or something else that I'm just not inclined to explain as part of an on-the-spot rejection.
So what do you think? What's the most polite way to handle it when you realize close to the start of an interview that a candidate isn't right?