Here is a short lesson on the unison line from Israel Houghton’s reggae song Surely, Goodness and Mercy.
It is a 4 bar phrase, the 1st two bars are the line and the 2nd two bars are the “answer”. The first answer is from the recording, the second one I created. After you grasp the vibe try coming up with your own answer phrases. Try and make sure there is contrast to the unison line and that you make use of space, playing melodically, playing off of beat (not always necessary), and note placement. Those of you familiar with modes notice how bar 2 almost plays the E mixolydian scale note for note except the 2nd and 4th degrees.