How a former British paratrooper prepared actors in '1917' to fight WWI's devastating battles

"Foot care was one of the first lessons I taught George [MacKay] and Dean [Charles Chapman], the importance of looking after their feet daily," Biddiss said, referring to two stars of "1917." "Basic recruits are taught this still even today."

Trench foot, a common condition in World War I, is caused by wet, cold, and unsanitary conditions. It can be avoided by keeping the feet dry and clean, but left untreated it can lead to gangrene and amputation.

"The boys were wearing authentic period boots, walking and running in the wet mud all day and if not addressed early would have cause them major problems on set," Biddiss said. "I taught them how to identify hot spots on the feet where the boots rubbed, taping up those hotspots to prevent blisters and applying talc and clean socks at every opportunity."

"I naturally train most days to keep myself in shape" and to instill confidence in his abilities, Biddiss told Insider.

"It's not a good look if you're a military adviser and you're carrying around excess weight" and get winded after a short walk, he said.

"We first ran local auditions," Biddiss said. "I then ran assessments before boot camps to make sure we had the right people who not only looked right, but were coordinated and physically robust to take on the task."

After the performers were selected, "I started with basic arms drill to test coordination, fitness to test stamina," he said. "Then to weapon handling, historical lessons, and tactics."

"There so much attention to detail, like I've never seen before on set," Biddiss said.

Weapons handling is one of the main hurdles in preparing an actor for battle.

"There could [be] over 500 supporting artists on set with bayonets fixed and firing blank rounds," Biddiss said. "The blanks used are very powerful and can still do permanent damage, so if time is not invested in training it could all go horribly wrong."

It's also one of the things he notices other productions often don't get right. Biddiss said he notices performers never reloading their weapons or always having their fingers on a gun's trigger.

"I like to impress on one aspect," Biddiss said. "Fear and anger."

"I tell actors and supporting artists that they need to show both feelings on their faces when about to act a battle sequence," he said. "Fear of dying, but anger towards the people who have brought them to this situation."

"There is nothing ninja about soldiering," Biddiss tells the performers he trains. "You have one job. Close in and kill the enemy."