I was not an extra myself, though I once went on a tour with a battlefield guide who was, and he told a couple of stories.
There is a scene early in the movie where Confederate officers, mounted on horseback, are discussing what to do next as what appears to be an endless stream of extras dressed as infantrymen passes by behind them. The guide was one of the infantrymen in the background.
The guide said that there were supposed to be hundreds of extras to participate in that scene, but when it came time for the shooting, only about 50 had shown up.
So the director simply had them march in a circle - behind the mounted officers, around behind the cameras, back behind the officers again, and so on.
The guide said he recognized his own straw hat passing through the picture three or four times during that scene.
Then, later in the movie, there is a scene where Martin Sheen as General Lee rides past the troops preparing for Pickett's charge. On the screen, you see all the extras (this time, many more than fifty) cheering wildly, and you hear "Lee! Lee!" over and over again.
When the scene was shot, the extras were told to cheer as loudly and enthusiastically as they could, but they were also told that the sound would actually be dubbed in later. They could yell anything they wanted to, since what they actually yelled would not be heard in the movie.
So when you watch that scene and you see those extras cheering, half of them are actually yelling "Tastes great!" and the other half are yelling "Less filling!"