They can hear and gauge their own voices.
They have the same ears as their listeners.

But the picture producer holds to his eyes the seven-leagued demon spy-glass called the kinetoscope, as the audience will do later.

The actors have not the least notion of their appearance.

Also the words in the motion picture are not things whose force the actor can gauge.

The book under the table is one word, the dog behind the chair is another, the window curtain flying in the breeze is another.
This chapter has implied that the performers were but paint on the canvas.