The actor spent three and a half hours every morning having sophisticated prosthetics applied before walking onto the "Darkest Hour" set during the 54-day shoot.
New film, "Darkest Hour" tracks a few weeks in the summer of 1940 during the Nazi invasion of France.
Winston Churchill had just become Prime Minister and was faced with deciding whether to continue fighting or negotiate with Adolf Hitler and Nazi Germany.
Actor Gary Oldman spent three and a half hours every morning in the makeup chair having sophisticated prosthetics applied before walking onto the "Darkest Hour" set during the 54-day shoot.
He arrived in the studio at 3 a.m. for makeup and the costuming took another half-hour.
"There was a guy called Kazuhiro (Tsuji), who's a Japanese prosthetics artist, who is a genius. And he's actually given up doing prosthetics and become a fine artist, and then Gary persuaded him out of retirement," says the film's director, Joe Wright.
"We went through many iterations of the makeup until we hopefully found that sweet spot where, you know, he looked enough like Churchill, but still allowed the audience access to his performance."
Removing the prosthetics took as much as two hours and could not be removed quickly without damaging Oldman's skin.
He also wore a foam body suit to help recreate Churchill's shape and posture.
Oldman revealed his Churchillian makeover during the four-week rehearsal period before the shoot began.
"Churchill walked into the rehearsal room, so it was... it was incredible," says Lily James, who plays Churchill's secretary, Elizabeth Layton.
Oldman, who was Oscar-nominated for "Tinker Tailor Soldier Spy," has previously transformed himself into characters as diverse as Sid Vicious and Lee Harvey Oswald.
Director Joe Wright was taken aback by the way Oldman adopted Churchill's distinctive speaking voice, and when he heard the 59-year-old British actor make the famous "fight them on the beaches" speech, he was both relieved and overcome.
"I felt quite emotional, actually. I think most people in the room at the time felt quite emotional," says Wright ("Pan," "Atonement".) It was a very touching moment, you know." said Wright.
Churchill himself is among the most revered and studied figures of 20th-century history and his speeches of May and June 1940 mobilised a nation.
"Churchill first and foremost was a great, great writer, and that was the genesis of the project: you know, how did someone write - or why and how did someone write - three of the greatest political speeches of history in this short four-week window." added Wright.
"Darkest Hour" takes place almost exclusively indoors during parliamentary sessions and private meetings.
It offers the diplomatic side to the recent action movie "Dunkirk," Christopher Nolan's recent film which follows the evacuation of Allied soldiers.
Cut off and surrounded by the German army in the French city of Dunkirk, Churchill decided to risk evacuating them by sea.
"Darkest Hour" gets its title from the understandable depression that had spread throughout the British government at that time.
Members of Churchill's new War Cabinet wanted to negotiate for peace.
Churchill's predecessor Neville Chamberlain and senior conservative politician Edward Wood both considered Churchill unhinged for believing Britain could survive.
As the film notes, Hitler may have had the world's greatest army in the spring of 1940, but he still had no way of transporting it across the British "moat" of the English Channel.