The members of an orchestra, a band, or a choral ensemble will constantly be watching each other, waiting for cues, listening for pitches, testing for harmony.
When we sing psalms and hymns together, we look anywhere except at each other. The ceiling, the floor, a far-off celestial city, even if we have the words memorized we do not meet one another’s eyes. Are we singing together? Or are we each praising God by ourselves in our own little musical bubble?