Behind and By and by

Behind adverb - In or to or toward the rear.

By and by and behind are semantically related In some cases you can use "By and by" instead an adverb "Behind".

By and by adverb - At or within a short time.

Behind and by and by are semantically related in event topic. Sometimes you can use "Behind" instead an adverb phrase "By and by", if it concerns topics such as order, later, afterward, happening after.