That doesn’t necessarily prevent the people from answering greetings or making other responses. In the Byzantine Divine Liturgy, the nave is separated from the altar by the iconostasis. When the priest is greeting the people (“Peace be with you”), the doors are open, the priest is visible and facing the people. He greets us, we respond. As I recall, during the Tridentine Mass, the priest also turns around to face the people.
Was that not the case in the Latin Rite, even if the altar was not visible to the people?