Ballad scholar David Buchan suggested that at one time there were no set texts for ballads, just plots and commonplaces (milk white breasts, snow white steeds, wee penknives etc) and that a singer would extemporise every time he or she sang.
The nearest I have seen this in practice was with MacColl as he grew older and his memory started to go.
He would never dry up on a song, but quite often you could spot changes in his texts. You would always know he had put something in because he had forgotten the written text, by the little grin.
Jim Carroll