From the description of things, this was very whim-ish. According to her camp, Swift really did write the song just this past Sunday, December 1. It sounds less spontaneous than that, given the strings and gospel singers that appear on the tune, which Swift wrote solo and co-produced with Jimmy Napes (best known as Sam Smith's collaborator). But if anyone can acquire a choir and orchestration on the spur of the moment, it would be Swift -- who apparently was not fictionalizing the part about missing home, since credits show she recorded it far, far from firs at London Lane Studios in the UK (which is at least a home for the transatlantically smitten singer for the holidays).