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"You are the dead," repeated the iron voice.
"It was behind the picture," breathed Julia.
"It was behind the picture," said the voice. "Remain exactly where you are. Make no movement until you are ordered."
It was starting, it was starting at last! They could do nothing except stand gazing into one another's eyes […] unthinkable to disobey the iron voice from the wall. There was a snap as though a catch had been turned back, and a crash of breaking glass. The picture had fallen to the floor uncovering the telescreen behind it. (2.10.21-24)
Winston and Julia come to a rude awakening when it turns out that their rented room has a hidden telescreen that has surveyed them for their entire affair.