 PLOT: For much of the flight, and in particular during the launch  when the astronauts have not yet been informed of the deception,  were told their voices from the spacecraft are simulated by  recordings that were made during the practice simulations. This  trick would stop working as soon as anything was said to them  requiring a reply that had not occurred in the simulations, or as  soon as they had any quiet time and would naturally be expected to  chat about their present experience.