A great deal of the dialogue is improvised. This was a deliberate choice on the part of Sir Sam Mendes to be a little more organic after the stylization of Road to Perdition (2002).
The sex video breakup scene is actually a well-known legend that has been circulating in the American military since the late 1980s, and actually does happen.
The burning oil wells were all computer generated. The oil that appears on the soldiers' faces was a concoction made from molasses.
The interviews with the grunts were all improvised. Lucas Black was particularly uncomfortable with this, as he preferred to work off a script.