The devil is in the detail
Just imagine being in a cinema watching The Blob. On the screen in front of you is a cinema with "you" sitting amongst the audience. The camera switches to the back of the cinema. By the projection box are long slats, presumably part of air conditioning (if it existed then) - perhaps just ventilation. Suddenly something appears coming through the slits (between slats). The substance joins up this side of the slits. It grows bigger very quickly - a shapeless mass just getting larger and larger behind you in the audience. in a sudden rush of shock/fear you, and all the audience, turn rapidly to see the imminent danger . . . . . . . . . which isn't there in the real cinema.