An invasion calls for a sizable number of armed forces, and when you need armed forces, the best place to turn is the military. The Irish army provided 750 extras for the D-Day scene, many of whom were movie veterans, having worked of Mel Gibson's "Braveheart."
Feeding and costuming so many extras might have been a logistical nightmare were it not for a system dubbed "the sausage machine," originally perfected by associate producer Kevin De La Noy when he also worked on "Braveheart." The extras were broken down into 15 groups of 50, which were fed, clothed, and made up in varying order. At the end of the day, each group went back through the system in reverse. It worked like a proverbial well-oiled machine.