How accurate is the dialogue style in '12 Years a Slave'?

Well, I will say that the way people spoke in the movie is the way people spoke in the book. Many lines were taken straight from the book, which was of course a true story. One thing that was interesting in the book is that surely Northup, from upstate New York, spoke differently than the people in Bayou Boeuf Louisiana, where he spent most of his time as a slave. But virtually no one comments on it, or at least, he does not mention anyone commenting on it in the book. Just after he is captured and enslaved, he is told (in Washington DC) that he is to tell people that he is from Virginia and has always been a slave there. Yet Virginians must have spoken differently than back country Louisiana people did, too.

Here's another anecdote: Sojourner Truth, a woman born and raised a slave in upstate New York and only about 10 years older than Solomon Northup, was a very prominent figure in the abolitionist movement (as well as women's rights). She gave a very famous speech that was widely recorded and repeated. But here's the thing: in the first reportings of the speech, she spoke in a way that you would describe as like Shakespeare (in truth neither she nor the people from the movie 12 Years a Slave spoke in a Shakespearean way, but I'll leave that aside). But in subsequent reportings of the speech, her language had been completely changed so that she spoke in a way that sounds more like what has become the stereotype for how slaves spoke, and in fact the speech has become known as the "Ain't I a Woman" speech, even though she never said those words. Another example: Frederick Douglass was an escaped slave from Maryland who was about 15 years older than Northup, and who lived in upstate New York. He wrote in ways that sound like what Northup's speech was in the book and film. People had trouble believing that a slave could sound like that or write like that, but he did. He was completely self-educated. The point is, yes, many people spoke in that highly refined way, slaves included, in New York of that period, and Northup was better educated than most Americans at that time, knowing how to read and write and play music. But that way was not the stereotypical way people of that time, or this time, thought of slaves speaking. And that is so much true that slaves' (and non-enslaved black people's) words of that time have been altered, or called into question, because they did not sound as expected. But we have every reason to suppose that people like Northup did speak that way.

I realize this is not a complete answer to your question, but hopefully it adds a bit of context to why the makers of the film chose to have people speak the way they did.