This embellishment is by no means an isolated case in the film. For instance, in the film version, shortly after Northup is kidnapped, he is on a ship bound south. A sailor enters the hold and is about to rape one of the slave women when a male slave intervenes. The sailor unhesitatingly stabs and kills him. This seems unlikely on its face—slaves are valuable, and the sailor is not the owner. And, sure enough, the scene is not in the book. A slave did die on the trip south, but from smallpox, rather than from stabbing. Northup himself contracted the disease, permanently scarring his face. It seems likely, therefore, that in this instance the original text was abandoned so that Ejiofor's beautiful, expressive, haunting features would not go through the entire movie covered with artificial Hollywood scar make-up. Instead of faithfulness to the text, the film chooses faithfulness to Ejiofor's face, unaltered by trickery.
Other changes seem less intentional. Perhaps the most striking scene in the film involves Patsey, a slave who is repeatedly raped by her master, Epps, and who as a consequence is jealously and obsessively brutalized by Mistress Epps. In the movie version, Patsey (Lupita Nyong'o) comes to Northup in the middle of the night and begs him, in vivid horrific detail, to drown her in the swamp and release her from her troubles. This scene derives from the following passage at the end of Chapter 13 of the autobiography:
Nothing delighted the mistress so much as to see [Patsey] suffer, and more than once, when Epps had refused to sell her, has she tempted me with bribes to put her secretly to death, and bury her body in some lonely place in the margin of the swamp. Gladly would Patsey have appeased this unforgiving spirit, if it had been in her power, but not like Joseph, dared she escape from Master Epps, leaving her garment in his hand.
As you can see, in the book, it is Mistress Epps who wants to bribe Northup to drown Patsey. Patsey wants to escape, but not to drown herself. The film seems to have misread the line, attributing the mistress's desires to Patsey. Slate, following the lead of scholar David Fiske (see both the article and the correction) does the same. In short, it seems quite likely that the single most powerful moment in the film was based on a misunderstood antecedent.
Critic Isaac Butler recently wrote a post attacking what he calls the "realism canard"—the practice of judging fiction by how well it conforms to reality. "We're talking about the reduction of truth to accuracy," Butler argues, and adds, "What matters ultimately in a work of narrative is if the world and characters created feels true and complete enough for the work's purposes." (Emphasis is Butler's.)
His point is well-taken. But it's worth adding that whether something "feels true" is often closely related to whether the work manages to create an illusion not just of truth, but also of accuracy. Whether it's period detail in a costume romance or the brutal cruelty of the drug trade in Breaking Bad, fiction makes insistent claims not just to general overarching truth, but to specific, accurate detail. The critics Butler discusses may sometimes reduce the first to the second, but they do so in part because works of fiction themselves often rely on a claim to accuracy in order to make themselves appear true.