One takes the storyboards and makes a story real with the sound track. So you record both actors, animating later the sound track frame by frame, so you know exactly which syllable, which letter is sitting on which point in time, frame by frame.
Many motion pictures in the 1930s and 1940s relied on production sound—that is, all the dialogue and extra sounds recorded live on the set. Soundstages were quiet, controllable environments for recording not only “clean” dialogue, but also footsteps, door slams, or other sounds necessary for the scene. However, there were particular sounds that couldn’t be created live. War movies, westerns, gangster films, horror, or boxing films ...