On 1 May 1977, John Stark attended a preliminary screening of a new "sci-fi" film called Star Wars. He sat behind the director, George Lucas. Stark retells the story wonderfully on Business Insider:
At the end of the film the audience went ballistic. They couldn't stop applauding. I watched Lucas slump in his seat, overwhelmed, as if he’d just outrun the Death Star’s super laser. Attendants tried to collect the preview cards, but nobody was filling them out. There was no need.
(I later read that Fox President Alan Ladd Jr. had flown in from Hollywood for the screening and that he was moved to tears. He had green-lighted the film two years earlier and his reputation was on the line.)
The next day at work I tried to describe to my colleagues what I had just seen. I told them about The Force, Darth Vader, Obi-Wan Kenobi, the cantina on Tatooine. “Star Wars is going to be The Wizard of Oz of our generation,” I proclaimed. They thought I had lost my mind. I shut up.
Follow the link for more. It's a great read.