#1 Margaret Hamilton's Makeup
Margaret Hamilton's makeup was toxic. To keep from accidentally ingesting it, she had to stick to a liquid diet for the whole filming of the movie. That's not really surprising. Anything that green can't be good for you. Still, that's a long time to go without food. The movie took between 5 to 6 months to shoot. Shoots lasted most of the day. That's commitment. At least her character was supposed to be grumpy.