Director Chris Eska mentioned (at 0:01:05 in the DVD commentary) that the flashes of distant cannon fire in the opening scene are visual effects that in real-life would precede the sound of the cannon fire by several seconds, but when the correct time delay was used the screen looked strange, so the filmmakers made the flash and the sound appear simultaneously to avoid throwing the audience off from the beginning. The flashes were about 3 seconds apart. Since sound travels about 1 mile in 5 seconds, artillery 3 miles away would have a 15 second disconnect between the flash and the sound.
Director Chris Eska has a history of working with non-professionals, so in searching for someone to play the role of the boy Will, the filmmakers looked extensively at every school within 200 miles of their base in Gonzales, Texas, and auditioned four to five hundred kids locally (at 0:35:50 in the DVD commentary). Regardless, they found Ashton Sanders in "the most boring way possible, which was through a regular casting call" in Los Angeles. Ashton's agent submitted him and he came to at least two callbacks (at 0:48:00 in the DVD 'SAG Foundation Conversations').
When Will is going from man to man in Burrell's base camp serving beans, he slips something from the duster pocket of one of the men he serves (at 0:07:03). Closer examination on the DVD confirms it is the pair of cigarettes he then sells to the next man he serves. This 3 second act alerts the viewer to Will's moral compass.