They also did a lot to get the speed to look more natural.
The director said that they expected all the archive film had been shot at 16 fps, but they were surprised to find it had been shot at many different speeds including 11, 12, 14, 16, 17, 18 fps. Something as simple as frame rate had not been standardized in the teens. They had no archival data to use to play the film back at the correct speed, so they experimented and found they could restore it at one frame too slow and one frame too fast, but neither looked right. He said they knew when they got it right. At the end, he showed clips one frame too slow, one frame too fast, and just right. It was indeed obvious which was correct.
His attention to detail was just astounding. They looked at regimental markings to learn where a unit came from. Then, they would use oral histories by men from those unit hometown shows to narrate the movie!
The director didn’t want to use New Zealanders to sing a WW I song at the end, so he went to the British consulate and got men who were amateur singers to si g the song. All for authenticity.