In the chapter where you must find Capt. Keyes in the swamp, there is a secret area you can access right from the beginning.

When Foe Hammer says “The Last transmission from the captain's drop ship was from this area. That was over twelve hours ago.” immediately start pressing your grenade throw button. Whatever you do, DON'T MOVE! After a few seconds, the drop ship will take back off, and you'll still be in it. It flies for about 20 seconds then crashes. Press X to exit the drop ship.

Now walk directly away from the ship until you reach a clearing. Head towards the center and out of the fog you'll see a lone marine standing there! He won't move or say anything, unless you shoot him in the foot w/ your pistol.