My guess: For the first version of the scene they’ll have someone saying the other character’s lines the way they should be played so that dialogue looks natural. Once one version has been filmed it’s not hard to play that version’s audio while filming the second version so the actor is actually hearing himself play the other character.

On a virtual meeting a guy unknowingly had his mic on and shouted to his kid “N…r sit your ass down!”

Ok now show the stat for alcohol!