I was on a United DC-8 in 1971 when I watched them install the inflight movie. I was flying RIC-IAD-LAX. On our stop at IAD, a union projectionist from the Washington local got on the plane with three huge reels of 16mm film, one for first class and two for coach. He then pulled down the projectors from the ceiling, which hinged down, and loaded the reels horizontally into the projectors. Then he pulled down the CinemaScope screens (wide screens) from the ceiling and started running the films so he could make sure they were properly focused and aligned with the screens. That way the flight attendants would only have to turn them on. Then he wound them back to the beginning and retracted the cabin screens. When they ran the film during the flight, we had to close the curtains over the windows for better viewing. I was near the rear of coach, and I noticed that the movie in the forward coach section was a little ahead of ours.
When I flew on Eastern L-1011s and A300s, I discovered that their movies were on Super 8 film, and of much poorer quality than United's 16mm. They were out of focus, and the flight attendants didn't know how to correct this.
Now, most airlines use video projection or TV monitors.
Fly Eastern's Golden Falcon DC-7B