"I'm sure the understudy does NOT get personally affected by any reaction (though I don't ever remember hearing one); they KNOW they are the understudy and that they only get to perform when necessary. Any parent or friend in the audience that doesn't understand that is being unrealistic. "
haha its obvious we disagree on this, but i want you to know im enjoying the banter.

I dont think its being unrealistic that the people who came to see an u/s are disappointed by audiences reactions. Just like people go to see nathan lane, people go to see understudies. And their experience in the theatre is just as important i think.
And u/s are just people, not robots. I feel like saying they don't get personally affected is like saying the last kid picked at dodgeball isn't either (similar emotion not situation). Sure they signed on to be an u/s. Sure they know that they go on in the event that someone can't. But that does not change the fact that they have feelings and can perceive (albeit misconstrue) an audience's grumbling at the announcement of their names.
And again im glad youve never experienced an audience doing this, Butttt i have on many occasions.
It just sounds so ungrateful to me. And its largely due to people not understanding the inner-workings of theatre. Without that u/s going on, those groaners would be standing on the street corner looking for something else to do for the night. Wondering how they can reschedule or get their money back, and how much of a hassle this is going to be, or complaining that they are only in town for one night... etc. But instead they are in their seats, watching a u/s work their butts off, just so that people dont come on here and rip them apart for not being Karen Olivo.