there are two distinct senses given by most sources for prosopopoeia: (1) a personification or embodiment of a quallity or other abstraction [this is what we usually think of when we think of personification]; and (2) an absent [read dead] or imaginary person is portrayed as speaking or acting -- usually where a plausible but invented speech is put into the mouth of a real character.

It is this second sense which seems to be giving us problems. "I am dying, Egypt," says Antony to Cleopatra in his death scene of their eponymous play -- obviously we have no recording of this ever being said, but the prosopopoeia is dramatically justified! E. L. Doctorow made a writing career of using this device in his fiction.