"you need to know the subject better than the person that you're interviewing. If you don't, you have no hope of screening for good people."
I think it is not necessary and not possible that the interviewer know more about every subtopic than every candidate. It is enough that they have comparable knowledge in most topics. Good candidates can very easily know more about some parts of the subject (and maybe less about other parts). Being a good interviewer is a really tricky business, because good interviewers should see the strength of candidates and should see if the candidate know even more than them about some subtopics. I've seen some bad interviewers who asked only the very narrow topics they knew well, and basically wanted exact copies of themselves.
I think it is not necessary and not possible that the interviewer know more about every subtopic than every candidate. It is enough that they have comparable knowledge in most topics. Good candidates can very easily know more about some parts of the subject (and maybe less about other parts). Being a good interviewer is a really tricky business, because good interviewers should see the strength of candidates and should see if the candidate know even more than them about some subtopics. I've seen some bad interviewers who asked only the very narrow topics they knew well, and basically wanted exact copies of themselves.