Many of the extras who appeared throughout the film were local townsfolk who were recently left unemployed when a nearby mill had ceased operations, and they were more than happy to have the cast and crew of the film there to provide them work opportunities.
The large piece of rotten canvas that Rambo finds in the woods and cuts into a makeshift coat was in fact not a movie prop, but a real piece of rotten canvas found by the film crew during the movie's production. Since there was only one piece, Sylvester Stallone joked about how the canvas became a treasured prop on the set. After filming ended, Stallone kept the rotten canvas and still has it in his possession to this very day.
Sylvester Stallone accidentally broke the nose of Alf Humphreys (Lester) during the jail escape scene by elbowing him in the face, which is why he is seen wearing a band-aid throughout the rest of the film. Coincidentally, this is what Rambo does to a policeman in the novel during the exact same scene.
A plot point that was present in the novel but absent from the film was the primary reason behind Teasle's resentment and contempt toward Rambo, which was that Rambo was a veteran of the Vietnam War, which gained a lot of attention, whereas Teasle was a veteran of the Korean War, which most people had all-but-completely-forgotten at this point in time.
David Morrell, the author of original novel, stated that he prefers this film over his novel.