Beyound the obvious, an experienced (=good) interviewer has the following skills:
- Have the case running in any direction, without sticking to a pre-defined solution, depending on how the candidate sets up the structure and thinks about the question (it's not always right or wrong in a case interview - oftenly it depends on how you interpret some data, and I don't like to redirect candidates if they have solid argumentation into a specific direction).
- Challenging candidates' thought process and decision-making rationale, even if it's going into the right dimension. By that also making sure that it wasn't a lucky punch, but a clear solid thinking which led the candidate to some conclusion.
- Differentiating between not being comfortable with numbers and making a simple math calculation mistake. The big picture counts, obviously it's not good to make math mistakes, but I don't care if I see that you are doing well with numbers and have made somewhere a small mistake in a calculation (this topic is way to over-hyped..).
- Building high-pressure situations within the interview without being offensive towards the candidate. Yes, it's part of real-life situations that's why it's absolutely legitimate to test a candidate under pressure - but there is no point in being personally offensive towards a candidates.
Hope this helps - if so, please be so kind to give it a thumbs-up with the green upvote button below!