In The Song of Achilles, Miller walks the line between historical fiction and mythical retelling, bringing the divine powers of the ancient Greek pantheon to life, but not including other, more fantastical elements of the story (such as Achilles's vulnerable heel). She inserts her own interpretation of Achilles and Patroclus's relationship, interjecting more intimate moments between the two that neither myths nor histories include. Miller emulates famous Christian apologist and novelist C.S. Lewis, whose book Till We Have Faces retells the Greek myth of Cupid and Psyche, albeit from the perspective of Orual—a minor character in the original tale. Similarly, Miller chooses to retell Homer's Iliad not from the perspective of Achilles, but rather through the eyes of Patroclus.
As a retelling of The Iliad, focusing on Achilles and Patroclus's romantic relationship, The Song of Achilles is first and foremost a tragedy. The novel contends with intertwining threads of fate, free will, choice, and destiny. The Greek gods in this novel are cold, judgmental, and distant, prophesying death and destruction for Achilles and Patroclus while they remain sequestered on Mount Olympus, immortal. Perhaps the greatest tragedy of the novel is Achilles and Patroclus's loss of youthful love and innocence at the hands of such gods, who are intent on making playthings of mortals and demigods alike.