They didn't make up their lines, but they sounded as if they did, and they didn't at all seem to be acting. Today, actors are more natural, and almost always seem to be acting. Even when telling a lie, the voice has something true in it. A bad actor or singer can still be moving. A bad musician or painter can't.
Some actors like to eat on camera, and it's often done well.
But if you' re obliged to watch the motion of the actor's jaws and swallowing reflex in too much detail, eating on camera can be disgusting. It isn't so much that eating itself can be disgusting, but that the mixture of acting and really doing the thing, as in pornographic movies, can be disgusting. Mainly it's actresses who are shown asleep. They look good with their eyes closed. Men look dumb or dead.