A bit off-topic, but I thought y’all might enjoy this story:
Movie theaters do indeed come several days in advance of their release. The theater receives one or two canisters filled with 20-minute reels, however many it takes to make the full movie (a 100 minute movie takes 5 reels). The manager/projectionist/whoever will then splice the film together on the “platter”, the huge horizontal reels that hold the film for the projector. And, because this process is usually easily completed well before the release, the managers will indeed “preview” the movies themselves, to make sure everything’s okay. Usually it’s just the managers, but sometimes they’ll bring family members or invite other employees.
One such screening occurred at the theater where I worked before the premiere of Gremlins 2: The New Batch*. The managers were relaxing, having a fine time watching the movie, when it reached that point where, you’ll recall, there’s a rather realistic depiction of movie film snagging in a projector, getting trapped against the lens, and burning. The manager for that theater hollered, “Oh SHIT!” and bounded up the aisle to the projection booth to save the film. Before he made the door, though, the shadows of gremlins appeared on the screen, and he figured out he’d been tricked.