I grew up in a time (the 1960s and 1970s) when commercial, technological and artistic conventions accustomed us to listening to music on LPs and watching movies in theaters. For the most part, we listened to one side of an album at a time (eventually, CDs -- although more easily programmable -- would play 70+ minutes of uninterrupted music, which changed song-sequencing priorities). And we saw movies from start to finish. I'm too young to remember the original ad campaign for Alfred Hitchcock's "Psycho" in 1960, but later on (when I got my hands on some original lobby cards -- those were the 11"x14" images displayed with the posters at the entrances or in the lobbies of theaters) I noticed it was built around the apparently novel pitch that audiences had to see the movie from the start.
Now, if, like me, you were in college (or university, as they say back East) when Alvy Singer (Woody Allen) in "Annie Hall" announced that he had to see a picture "exactly from the start to the finish," and you thought that made perfect sense, it seemed bizarre to imagine a time when people had to be encouraged to show up before the feature started: "No one... BUT NO ONE... will be admitted to the theatre after the start of each performance..." (It turns out Paramount had done something similar with Hitchcock's "Vertigo" just two years earlier: "It's a Hitchcock thriller... You should see it from the beginning!") As the proprietor of the Opening Shot Project, which emphasizes the importance of the first shot in setting up and framing certain films, the idea that somebody would watch a movie without having seen the beginning is incomprehensible to me. Why cheat yourself of the joys of discovery and development? Or just knowing what's going on in the story?