Great post from Brian, clarifying many things. As far as I'm concerned, it's more that the singer is at the service of the story -- rather than that the singer is the vessel for the ballad. This allows for all the artistic discretion that Brian describes -- pace, decoration, volume, tone etc.
But I would rather this were not an 'artistic' effect, which is not what Brian is describing. There is really no way to sing ballads other than by knowing them (which implies some sort of apprenticeship).
However, I have to confess that the first ballads I sang - in the late 1950's - were only the start of a learning experience. Before 1960 I was singing 'The Twa Corbies', 'The Baron of Brackley', 'Sir Patrick Spens', 'Son David' and 'The Great Silkie' -- but I freely acknowledge that this is an ongoing process, and that my interpretation of these (and ballads later acquired) is in permanent flux. Interpretation varies depending on the physical circumstance, the audience's requirements/expectations, and my own mood at the time.