'1917' stars George MacKay, Dean-Charles Chapman reveal challenges of filming ambitious movie

CHICAGO (WLS) -- Sam Mendes' new film "1917" is based on the experiences of his grandfather. The movie tells the story of two lance corporals sent to deliver a crucial message through a post-apocalyptic landscape behind enemy lines.

The film was created to look like one continuous shot, and there was plenty to rehearse before cameras rolled.

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"We began six months before shooting on an empty field which would become the trenches, scripts in hand, walking through the scene, sussing out how quickly we'd say the lines, how quickly we'd be moving, what would be coming in either direction all that stuff," said George MacKay who plays Lance Cpl. Schofield.

Actor Dean-Charles Chapman, last seen in "Game of Thrones," went searching for his family connection to the war.

"It's my great great-grandfather called David Henry Pierce, and he was actually in a book called 'The Western Front Diaries,' which is snippets of diary entries," said Chapman, who plays Lance Cpl. Blake. "His diary entry is in there. I've read that pretty much every day before stepping on set to get me in the headspace."

The director made sure that the nightmarish landscape was authentic as possible.

"The No Man's Land sequence was really difficult," MacKay said. "I think because of the physical tension and the environment we were filming in. What you see on screen is how it was. And it was a drop in the ocean as to what those men went through."