Sentence:  Performers still made their entrances from the wings of a stage, the stage of the film studio, only now the <WSD>fifth wall</WSD> division  between onstage and backstage was replaced by the division between the field of action in front of the camera, recorded as the image within the frame of its lens, and the field of inaction outside its lens, outside its frame of reference. 
Meaning of the word marked with <WSD> tags:  Any of various other divisions between groups of people in the theatre.