Much of the footage had never been seen, having sat in the vaults of London's Imperial War Museum for many years.
Peter Jackson considers this his most personal film, due to his lifelong fascination with WWI and the resonance he felt through his grandfather who died before his birth due to war injuries. Such was his interest in the subject that when production on the documentary began he already had a large personal collection in storage of WW1 uniforms and weapons to be used for reference.
The crew reviewed over 600 hours of interviews with over 200 soldiers, and 100 hours of original footage. Peter Jackson claimed a full year was spent just reviewing the material.
Sir Peter Jackson did not receive any fee for the film, as he made the film as a dedication to his great-grandfather who fought in WW1, and was also initially interested in making a film with literal 100 year old footage.
When Peter Jackson was approached to make this film he was allowed unlimited access to the IWM film archives and, as the film was a co-production with the BBC, also allowed access to their film and TV library. He was told he could do whatever he wanted with the project just as long as it was respectful and interesting. Asides from the impressive computer augmented colorization of the monochrome film (which Jackson says would have been even better had he been allowed more time) one of the things he wanted to sort out was for the film to play at a normal speed so the action seen appeared normal and not jerky and jarring. This was also accomplished by a sophisticated computer algorithm, that surprised even Jackson with how well it did its job when the film was being scanned.