Which version of the song do you think is most true to the song depicted in Impossible?
There's no straightforward answer to this; I was influenced by parts of many versions.
At the same time, though, I was initially very taken with the Simon and Garfunkel version that I grew up with. In this version, the man makes three impossible demands of the woman, and she replies not at all; she has no defense (she is silent) and so seems to be the party at fault.
That version tells us little about the man. To gain the information that he is an unearthly being, I went back to the Child ballad string of versions. In particular, readers might be interested in Version I, "The Deil's Courting," which includes this: "'I will ask ye questions three; Resolve them, or ye'll gang wi [going with] me." See the full set of verse here:
Note, in this version, that the girl replies to the Elf and defends herself, by demanding tasks in her turn. ("'I will ask ye questions three; Resolve them, or ye'll neer [never] get me.")
This influenced the joint version of the song that Lucy and Zach come up with together, later in the book.
If you read other versions, you'll see this motif -- the counterdemands from the womann that nullify the Elf's demands and protect her -- repeated in several of them.
A whole set of versions are available here:
P.S. Impossible comes out TODAY! So go get your copy now!