What singers notice when singing in step-wise motion is that they don't have to work too hard to stay in tune with the other singers. At the introduction of sudden skips of intervals of more than a third, however, more attention must be paid to proper breath support.
For example, the opening measures of "Joy to the World" are a descending major scale. The singers instinctively know that the first note is high and in order to reach it they must take a good breath and support the tone with their abdominal muscles. The opening phrase of "How Firm a Foundation" is quite angular. It starts with an upward skip of a 4th, repeated notes, then an upward skip of a 3rd, then a downward skip of a 6th. These skips stay in the outline of the chord and most singers can negotiate them and stay in tune with proper breath support.