well, as a filmmaker it is VEEERRRRRY hard to do that. It helps the actors because they can stay in character better. Hopefully they didn't do more than one or two "full takes" of the war scene. Each one probably cost $100,000 to shoot with all the choreography. I can't believe that they did it without crossing over into the crew section. I must believe that they did, and later they edited them out, and edited explosions and things back in. It was just too perfect.
The movie is absolute gold.