The Real Story: The movie is based on Solomon Northup's memoir, Twelve Years a Slave, which was published in 1853.
How the Movie Differs: The film does a good job of sticking to Northup's story. One key difference, however, is that, in the memoir, Mary Epps (played by Sarah Paulson in the movie) never goes into a violent rage against Patsey (played by Lupita Nyong'o). In the book, Northup acknowledges that Mary appears jealous of her husband Edwin's affection for Patsey, but she is never moved to violence or incites a physical fight between Edwin and Patsey. Northup does note that Patsey did suffer greatly in Edwin's hands, but this was due to Edwin's own rage issues.