As an example -- in my line of work, one task we perform is processing files, some small, some medium, some very large; and I wanted to make sure a candidate understood the concept of Maps in order to perform O(n) to create a map and O(1) to look up items (generally speaking, I'm sure it's not exactly that in practice), which aide in efficient processing (the age old processor vs memory tradeoff). This particular candidate got everything I asked immediately, and the interview was very short. The person was made an offer soon after and has gone on to learn what we've done, adapt, and also teach us some things that have made the software better.
And the moment candidates grok the concepts I'm after, I confirm that's what I was after, and move on, trying not to waste too much time on any given item. And if they don't get the concepts after a short while, I try to lead them there and see how they react (is it a lightbulb moment or a confusion moment, etc). These are all signs for me to determine if someone is right for the position.
Interviewing is a 2-way street of respect. The interviewer often has all the perceived power, but in my experience, a good interviewer eschews that perception right from the get-go and lets the interviewee know that this is more of a discussion than any kind of trickery or exam, even when it contains coding questions.
This is why I will never interview for Google or any other company that has multi-stage interviews and also employs time-wasteful, and sometimes esoteric, methods such as "How many jelly beans fit in a school bus?"