Blimey - An impossible question...

Given that no two performances are ever >exactly< the same, we have to concede there is a stochastic element at play here. So the question is about conscious modification. Obvious points here are venue acoustic/atmospherics - do you need to slow up or speed up the performance to fit the room? Pitch issues - how is your voice today? sometimes I crank stuff up a note or two to get more volume, but these are issues of performance rather than interpretation.

So: Interpretation of the music - has the tune changed? has the phrasing changed because of the way the words are set in a particular verse, ie is the stress pattern different, does the musical phrase subdivide, do you need to breathe? if so what is the dramatic effect of a break in the line? Should the breath be audible or silent?

Interpretation of the words. Presumably the words are more or less constant - have you altered anything since the last performance? If so, what emotive effect does this have on the song?

Personally, I tend to learn a song over a period of time and then let it do it's own thing. I have found though that you can ham stuff up a lot more in song than you can with spoken word. I don't really hold with the "let the story tell itself" approach - I think if you can act a bit you can help these old ballads along no end.

Cheers,
Jon