Sergeant Russell Schalk, who played the Army recruiter, is a real-life recruiter and combat veteran of the 1st Cavalry Division. Jennifer Lawrence asked him questions in character, and he responded as if he were talking to an actual recruit.
Jennifer Lawrence was originally turned down for the role of Ree for being "too pretty." She flew overnight into New York City, walked 13 blocks in the sleet to the casting office, and auditioned with a runny nose and hair she hadn't washed in a week. Lawrence won the role, and ultimately, her first Academy Award nomination (for Best Actress) at 20 years old.
For authenticity purposes, most of the supporting cast were locals. Ree's sister was from the area and indeed the exterior of her home in the film was actually her real home. The clothes everyone wore were the actual clothes of the locals too. Indeed, the townspeople were given brand new clothes in exchange for their old, frayed items of clothing to be used by the actors in the film.
In the book, the Gail/Ree relationship is elaborated much more. The director, Debra Granik, said that they filmed a lot the scenes on that Gail/Ree timeline that they had to drop from the final cut of the film out because of time restraints.
When Teardrop gives Sonny and Ashlee two baby chicks, these are not cute pets but a real asset once they've grown into chickens at 4-5 months. How valuable depends on their genders, which is hard to tell with chicks - if both are male there'll be meat but no eggs, while if one of each then these birds will produce more chicks.
The trivia item below may give away important plot points.
Two important elements from the novel were omitted for the film version: as a young girl (though her exact age is never specified) Ree is fed hallucinogenic mushrooms and raped by Little Arthur, which explains their tense relationship in the film. The other is the source of acrimony between Gail's husband Floyd--Ree and Gail have an intimate history.