About a few of my faves. Everything from IMDB
  1. In It's a Wonderful Life, Donna Reed's character was supposed to throw a rock at a far away window. They brought in a marksman to shoot it out because it would be a very tough throw, but she had played baseball so she nailed it all by herself.
  2. During the test screenings for Speed, it became clear that the movie would be a hit because people would walk backwards to the bathroom so they would miss as little as possible.
  3. A real Navy Corpsman was brought in for the medical examination scene at the end of Captain Phillips and she and Tom Hanks improvised the entire thing. (Okay, you probably knew that, but I think this is such an extremely underrated scene and this fact is super cool to me)
  4. And speaking of improvisation, both the pictionary scene and the finale "talking head" with Billy Crystal and and Meg Ryan in When Harry Met Sally were improvised.
  5. Before Psycho was released, Hitchcock brought the rights to the novel, and bought as many copies of it as he could find, in order to try to keep the ending a secret.
  6. The head of Universal Studios didn't like the title "Back to the Future," thinking that no one would see a movie with "future" in the title. He suggested "Spaceman from Pluto" in a note to Spielberg, who wrote back to thank him for such a funny joke.