I have a simple question: in a search, do the members of this list routinely interview inside candidates if they *somewhat* fit the job description? Obviously, if the person does not fit the ad and has no chance of being hired there is no point. But what if the person fits the ad in some ways -- but not all? It seems to me that there is no harm done if they are interviewed. What do others think, what have others done in this situation?
On the plus side, it may do no harm at all, and it may communicate that you are invested in the professional growth of this individual. On the minus side, it may communicate that you think that the individual is just one or two pieces shy of advancement. For example, let us say that you are seeking someone with experience in Green and Blue. This inside person has plenty of Green experience but no Blue at all, and no expectation of getting any Blue experience anytime soon. You know that you would never hire anyone without Blue experience. In this case it is a bad thing, because you are raising expectations; if the person doesn't get this vacancy, or the next one, then he or she will become embittered.
If I have eighteen candidates with Green and Blue, and 61 with Green only, then I would not interview this inside person. It's unfair to the committee and to the candidate. On the other hand, if I have three Green-Blues and 61 Green-only, I might make the inside person my only Green-only interviewee.
As an internal candidate, how would one know if the interview was offered only as a courtesy, or as a genuine chance of being selected as the best candidate for the position?
You cannot know. They might have said the nice things to be polite, just as they might have said them because they believe them. And it does not matter anyway. You have the interview, which means you have a chance at the job. You can't control what the other candidates do, or what they bring to the table. You can only do your best, and you note that you are committed to doing that. Good luck.