From Alex Bilmes, Paul McCartney Opens Up About Lennon, Yoko, and More, Esquire Magazine, Jul 6, 2015 @ 11:42 AM (H/t 3QD).
ESQ: So you don't find yourself moved, in the way the crowd is, by the emotional content of the songs?PM: Not all the time. You wouldn't be able to sing. You'd just be crying. But yeah, there are moments. I think it was in South America. There was a very tall, statuesque man with a beard, very good-looking man. And he had his arm round what was apparently his daughter. Might not have been! No, it was, it was clearly his daughter. I'm singing 'Let it Be' and I look out there and I see him standing and she's looking up at him and he glances down at her and they share a moment, and I'm like, "Whoa!" [He shivers.] It really hit me. It's hard to sing through that. You see quite a bit of that. If I ever spot anyone crying during 'Here Today', that can set me off. I mean, on one level it's only a song and on another it's a very emotional thing for me. And when I see some girl totally reduced to tears and looking at me singing it catches me by surprise. This really means something to her. I'm not just a singer. I'm doing something more here.
From my book on music, Beethoven’s Anvil, p. 98 (where I'm talking about a performance by Bette Midler I'd mentioned earlier in the book):
I have had similar experiences while performing on my horn. Tears would well up in my eyes and I could feel a lump in my throat. If I gave in to the impulse I would be unable to continue playing. But if I tried to suppress it completely, the magic would be gone and my playing would become ordinary. I learned to bear down in my chest and abdomen “just so” and skate on the edge. The feeling didn’t disappear, but I could continue playing my instrument.
We’ve all had similar experiences quite independently of music. Imagine you are in some public place and you receive bad news, perhaps about the death of a loved one. You are stricken with grief and feel a strong impulse to cry. At the same time you feel a contrary impulse to remain reserved in public, to suppress the sobbing and the tears. Later on you are called upon to deliver a eulogy at the funeral. Once again you are torn. In order to speak intelligibly you must remain in control of your vocal apparatus. But you are speaking of your dead friend and so are also moved by a grief that wants to commandeer the same muscles in the service of crying out.This is not an unusual situation, nor is grief the only occasion for such conflict. Laughter, anger, and physical pain can also generate impulses we struggle to suppress. What are the parties to this conflict? They must be inside us, in the nervous system, but where?The impulse to cry, or laugh, or shout in anger, arises in subcortical structures and acts on the muscles of the trunk, respiratory system, vocal system, and face through one set of pathways. The impulse to block such expression arises in cortical structures and acts on the same muscles through different, though often physically contiguous, pathways.