I'm always hopeful that the person I'm interviewing is more capable than I, and they very often are. But there's an interesting thing: the more expert a person is, the more willing they tend to be to say "I don't know" (often rapidly followed by informed speculation and a comment about how they would find out). Combining that with the fact that true experts usually prefer to operate at the limits of their knowledge means it's not hard to get such a candidate to say "I don't know".
It's certainly not necessary to operate at the same level as a person to get them to operate at the limits of their knowledge and ability. In fact, relatively inexperienced people have a knack for asking those "naive" questions that lead into very deep pools.
Edit: I should also mention that my interview style is not in the form of a quiz. It's a conversation. I think that makes a difference as well.
I'm always hopeful that the person I'm interviewing is more capable than I, and they very often are. But there's an interesting thing: the more expert a person is, the more willing they tend to be to say "I don't know" (often rapidly followed by informed speculation and a comment about how they would find out). Combining that with the fact that true experts usually prefer to operate at the limits of their knowledge means it's not hard to get such a candidate to say "I don't know".
It's certainly not necessary to operate at the same level as a person to get them to operate at the limits of their knowledge and ability. In fact, relatively inexperienced people have a knack for asking those "naive" questions that lead into very deep pools.
Edit: I should also mention that my interview style is not in the form of a quiz. It's a conversation. I think that makes a difference as well.