It's a really good movie, even though the film isn't as good as the book. Also, for a person who grew up in the south, it's easy to tell that a couple of the black women in the movie aren't from the south, or if they are, they've lost their true southern accent. One of the giveaways is the use of a long "I" sound. If they say "I" with an 'IE" sound, instead of the short I, then they're generally from somewhere other than the south.
Viola Davis, who plays Abaleen in the movie, is from South Carolina, but her accent is not true southern. In the movie she uses the long IE sound in most places, and that just doesn't ring true, if one was actually from the south and as old as Abaleen is supposed to be. Octavia Spencer, Minnie, is from Montgomery, Alabama, and her accent is much closer to true than Ms Davis.
BTW, it's also easy to spot someone who thinks they understand southern speech, or who has been coached by someone who thinks they do, but aren't really from the south. Nothing as irritating as a "put on" accent.