I thought up a scene for "North by Northwest", but we never actually made it. It occurred to me that we were moving in a northwesterly direction from New York, and one of the stops on the way was Detroit, where they make Ford automohiles. Have you ever seen an assembly line? They're absolutely fantastic. Anyway, I wanted to have a long dialogue scene between Cary Grant and one of the factory workers as they walk along the assembly line. They might, for instance, be talking about one of the foremen. Behind them a car is being assembled, piece by piece. Finally, the car they've seen being put together from a simple nut and bolt is complete, with gas and oil, and all ready to drive off the line. The two men look at it and say, "Isn't it wonderful!" Then they open the door to the car and out drops a corpse!— Alfred Hitchcock (1962)
Where has the body come from? Not from the car, obviously, since they've seen it start at zero! The corpse falls out of nowhere, you see! And the body might be that of the foreman the two fellows had been discussing ... The real problem was that we couldn't integrate the idea into the story. Even a gratuitous scene must have some justification for being there, you know!
Here is at least a partial transcript of the video.