Re the comments about guitarists etc joining in and directing the unaccompanied song - sometimes audiences/listeners join in, specially with a chorus song - and take over!! Jeannie Robertson had a technique for dealing with that; she said that the song should be like a burn flowing. Sometimes smoothly, almost still - then quickly and erratically as the water fell down small waterfalls. The key being it was unpredictable! The singer is expressing
the story and emotions of the song or ballad, and is not bound by conventional
written music rules!