When you start an interview you most likely have a list of questions. You write pluses and minuses or give points for every answer, but what do you do when the candidate shoots all the good answers? You just go to another point on the list?
I prefer to do it the way my mathematician did in the secondary school. “Well, that’s the right answer, how about going a bit deeper? Still correct… Do you know the law you’ve used here? You can… Can you prove the law? Oh, you can’t. No A this time. Sorry.” When you check one’s knowledge in specific area you won’t really know how deep the knowledge is unless you touch its borders. It has nothing to do with looking for holes in interviewees’ knowledge or trying to point their place. You just want to know what you’re going to pay for.
When you just go through standard questions prepared for average candidates you’re going to end with hires just a bit above the average if you’re lucky. You should rather seek for far better than average.
Whole avoiding hiring mistakes series.