I'm with Jim and his disquiet at the theory that singers brought nothing to the material but were merely passive recipients, and I like Jon Bartlett's reminder of another theory that there was such a thing as a 'rationaliser' singer -- which chimes with some interpretations of Anna Brown and her contributions to the ballad canon.

I've been singing ballads since the late 1950s and have always found that a completed back-story (often only in my own mind) was essential for a committed performance. I've been singing 'Lamkin' from around the late 1970s and the text I had was basically the Jamieson one with explicit mention of the mason not being paid but some omissions (mention of the lord sailing away and his warning, the whereabouts of the other servants etc.): but I was most taken by the unexplained hatred of the maid for the mistress, and found that I needed some way of making this real in performance -- so, for me, the lines that clinched it were 'What better is the heart's blood / O' the rich than o' the poor?', which became the pivot of the whole plot even to the point of suggesting to me that the nurse worked on Lamkin and poured her bile into his ear, working him up to do the deed.

I personally can't imagine a ballad singer persuading an audience unless (s)he has a clear notion of what the full story "might" be.

And, of course, there may be as many back stories as there are singers!