I recently began checking out the YouTube channel of the British Pathé, one of the largest historical video archives on the planet. Pathé news was filming nearly everything between 1910 and 1970 in the UK and around the world. They have a few recordings of theatre life in decades past. I really enjoyed this 1933 “Peep Behind the Scenes”:
Or how about this 1926 look backstage at the London Coliseum (there is no sound in this one)?
How’d you like to do scene changes while wearing a coat with tails?