Not to stomp on the positive vibes in the comments above, but Cornell was EASILY the most challenging interview I had. My MS4 interview was super relaxed/pleasant, typical of interviews I've had with med students, but I was grilled for a majority of my faculty interview. This was my last interview and even so, the 'big questions' I got were far different than anything I had encountered. That's not to say they were bad--I was super engaged! At one point my interviewer even said they'd stop beating down on me.... only to continue later on. 😆
That said, my interviewer was incredibly honest and transparent after the 'stress test' and it really stuck with me. I felt like I accomplished something coming out of the interview. It felt like the perfect wrap-up to my interview season.
So, to address the question above for future interviewees (at any school): it can be relaxed, or not. And if it's not, try to acknowledge that the harshness of the approach is not personal; they may simply be trying to see how you do under pressure, or how you think when you are being challenged. Take a deep breath and stay humble!