There is a lot of boll(plural French for water) here.

When a singer or player takes a song, they do it their way. Unless of course they are Sinatra, (or maybe Dino the Wino) in which case they do it a way that cannot be refused.

You work out how you are going to sing it and play it. Over time that evolves. Tempi change.   Introductory graces change - I was mucking about with "Nelson's Blood" tonight and noticed how I vary the introductory grace on each verse.

Then there is the "dog on the day" factor. This does not include the key (if accompanied) but will include little things. I am currently also working on "Sam Hall". I notice that because it is not (well, in my case) perfomred in tempo, the accents change in each playing.

The words, of course, you will have researched early on. You may have decided that a version does not ring true, and left a bit out, or truncated and elided verses. Thus I never sing the "Albert" verse in Radcliffe Highway. Similarly I have edited the (contemporary) "Princess Alice" to shorten it and I have also got the "Famous Flower of Serving Men" down to 19 verses.

Some bits of the way you do a song will be fixed before you do it. Others may be the subject of "on the night" inspiration.

Which is "interpretation"? In any event either is allowable; that is what folk singers do to folk songs - transmit them via the oral tradition and modify them.