One thing that's always struck me about that film is the dialogue. Most of the characters speak very formally with an impressive vocabulary, using few contractions. The Coen brothers used much the same dialogue as was used in the book (this is why it shares so many exact lines with the John Wayne version). Charles Portis (the book's author) supposedly did some sort of linguistic research when he was writing.

In this interview, Ethan Coen guesses that the formal language comes from familiarity with the King James Bible, based on the supposition that few people would have read or heard much else.

That doesn't sound convincing to me. American English in the 1870's was far from contraction-less. This little article has some good evidence, including analysis of the use of contractions in near contemporary books such as Tom Sawyer (which contains 226 instances of "it's", and 44 of "it is"). So the avoidance of contractions in common speech is bunk.

What about the vocabulary? Every character speaks as though they are legally trained, from LaBouef ("You could argue that the shooting of the dog was merely an instance of malum prohibitum, but the shooting of a senator is indubitably an instance of malum in se") to Lucky Ned Pepper ("What is your intention Rooster?") to Cheney himself ("I must think over my position and how I may improve it"). That doesn't really sound like the sort of word choice used by frontiersmen or outlaws.

Here's where I stray from historical fact checking into something more subjective. I've always thought that the dialogue was used as part of an allegory of sorts. One of the most important scenes in that film is the trial. (Parenthetically, this scene was filmed in the Blanco County Courthouse in Central Texas, which was built in the 1880's. So that's roughly contemporary to the film. It's also a lovely place to visit!). During the trial, Cogburn is put to task for how he achieves his brand of justice. The defense lawyer accuses him of being a bushwhacker, not a righteous office of the court.

But the realities of the frontier are... gritty. Things are dangerous, you can't trust anyone, etc... The thing about this film is that the entire story is one big courthouse scene. The characters all run around in the Choctaw Nation using formal legal terms like malum prohibitum and writ of replevin, they encounter each other in the wild, and they try to extract what justice they can. Mattie in particular is seeking justice for the murder of her father. Her knowledge of the law is the only thing that really helps her (for instance, the scenes with the horse trader). As the protagonist, she's basically a lawyer navigating through a series of 'trials' to see a criminal punished. That is why I don't think much historical credence can be given to the dialogue. It's all an allegory.