Even without greenscreen, actors in film and TV have to split their attention in unnatural ways. To preserve eyeline continuity in close-ups, actors often have to address themselves to a tennis ball or something near the camera lens, while their partner in the scene stands behind the camera. In the worst case scenario, sometimes the other actor is not even there when the closeups are being shot.
I remember an interview with Chevy Chase who said Dan Aykroyd was an expert at this and he said he was able to read cue cards out of his peripheral vision when they were at SNL so he could read lines without turning his head in the direction of the car holder. Chevy said he never met any actor that could do it as easily as Aykroyd could.
