Well masterclasses are an interesting set up: you (humble amateur/student) are here in front of a Master and a crowd (public masterclass) to play and be openly criticized... Both master and student are individuals with their flaws. They may not be meant to even like each other... and they are here, one dissecting the technique and musicality of the other and the player trying to be at best.
Can there be anything more intimidating?
I think some Masters understand that pressure and try to release it with jokes, anecdotes, etc. in the hope that the student will be as relaxed as possible. And this generally works if you - student - are willing to absorb as much as you can during the session. If you come with this mental preparation: be humble, be open minded, don't take anything personal, be relaxed, love the Master and be curious to ask anything, be curious to a point to even show some difficulties you have with technique or musicality and the experience will be awesome. Only then you can enjoy the moment.
I remember the first MC I attended was with Celino Romero and my hands were shaking to a point that I couldn't really play. Nothing to show in term of technical skills. He spotted that, talked to me, put me at ease and asked me to start over. I understood that I was not there to perform but more to learn and the second run went much better.
I always keep that in mind. Even if the Master appears to be mean, pretentious, even irrespecutful, I always think that his/her motivation is rooted in the deep knowledge and understanding of the music, nothing to do with the student. Yes Segovia appears to be a jerk on these famous Chaconne MC on Youtube vids... but he is the one who transcribed it, who adored it, who understood it down to the finest details. I can't find anywhere any remark that was not justified as none of the performer was bringing a true novel way of approaching the piece nor even a passage. His words may have not been the best (especially in English), his body language neither, but I am convinced that he wanted to convey his experience. And I think that only the open minded performers took away something, something that was not frustration.
Masterclasses are useful, a unique moment to learn.