Ben Zimmer details how the Oxford English Dictionary helped Tony Kushner, who wrote the screenplay for Lincoln, master the language of 19th century America:
One key to making the language historically suitable, he told me, was having the 20-volume print edition of the Oxford English Dictionary close at hand. A complete set of the OED—which includes deep histories of all its entry words, with examples—was one of his first purchases when he started earning money from his 1993 Pulitzer Prize-winning Broadway play, “Angels in America,” Kushner said. Through the many drafts of “Lincoln,” he checked every word that he thought might not have been appropriate for 1865.
Occasionally, browsing through the volumes led to serendipitous discoveries. Looking for a word for a White House party occurring early in the film, Kushner initially thought of shivaree, an American take on the French charivari. He was disappointed to learn that shivaree specifically referred to a wedding celebration. But then his eye landed on a nearby entry: shindy, just after the similar shindig. That word for a merry gathering fit the bill nicely.
The blog Languagehat, however, caught some linguistic mistakes in the film:
Lincoln pronounced the last word in the phrase "forever and aye" as /ay/ (as in "Aye aye, sir!") rather than the correct /ey/ (as in "A, B, C"). This is not a matter of dialect or idiolect; in the nineteenth century anyone who used the word would have said it in the only available way (which they would have heard in speeches and sermons, not learned from books). To quote the OED (in an unrevised entry from 1885): "The word rhymes, in the literary speech, and in all the dialects, with the group bay, day, gay, hay, may, way." The second was when Lincoln is telling his (truly hilarious) story about Ethan Allen going to England after the Revolution and being insulted; when he asks where the privy is located, Lincoln talks about his being directed "thence" when the appropriate word is "thither." Again, these are words to which dwellers of the twenty-first century are unaccustomed but that no one of Lincoln's day would have confused.