I think I posted something similar here a number of years ago, but couldn't find it in a search.
I went to see DREAMGIRLS at the Shubert Theatre in Boston, October 1981, before it moved to Broadway. I was in my early 20s. As fate would have it, the official "opening night" was supposed to be held on the night before I was seeing the performance...but one of the cast members (an unknown named Jennifer Holliday) had been under the weather so the actual "opening night" was coinciding with the performance I was seeing.
I had a great single seat; center orchestra about 6 or 7 rows back. Elliot Norton, "The Dean of American Theatre Critics," was sitting next to me on my left. Seated directly behind me were David Geffen, Phil Donahue and Marlo Thomas. Donahue's television show was huge then, so I knew I had a little story to tell my friends later on.
As the first act was heading to a finish, this unknown Jennifer Holliday (who'd already been quite good in the show's first act...and was the reason the "official opening" had been delayed until that night) began "And I Am Telling You I'm Not Going." It was like nothing I had EVER experienced in a theatre. The audience reaction practically blew the roof off of the Shubert. People rose to their feet screaming and applauding. David Geffen actually was standing on his seat! From, the mezzanine and balcony, audience members began tearing their playbills up which were raining down onto the orchestra section; improvised confetti! It was as if people, myself included, were losing their collective minds over what they'd just heard.
In the days before the Internet and show's previews being widely reported on, that night was made all the more impactful because nobody in that audience knew what was about to hit them.