Critics are already hailing the film adaption of Solomon Northup’s slave narrative, “12 Years a Slave,” as a shoo-in for the Academy Award nominations. But many viewers will likely wonder, given that this is a big-budget film with big-budget actors, how faithful it is to the book. The stakes may seem particularly high for a slave narrative, since many likely believe these books offer an unfiltered view of slavery.

But the question of authenticity gets the underlying medium wrong: Slave narratives were never intended to give an unbiased view. They were antislavery polemics meant to bring down the institution. Though many of the best narratives, including Northup’s, have withstood scholarly scrutiny and are essentially reliable, they still contain inaccuracies, distortions and embellishments.

That fact alone should give us pause in judging the film by a false standard of authenticity. If the film is “true” to anything in the book, it should be true to Northup’s voice, not his facts; that voice is what makes “12 Years a Slave” so enduring....