Despite the unusual casting, Gary Oldman has become the frontrunner to win most of the Best Actor prizes during this year's awards season for his performance as Winston Churchill in Joe Wright's Darkest Hour. He already has the Golden Globe on his mantlepiece.

Gary Oldman in 'Darkest Hour'Gary Oldman in 'Darkest Hour'

"I think my reaction was much the same as yours," Oldman says about picturing himself in the role. "A Churchill project came my way in 2014, and my reaction was, 'Don't be utterly ridiculous.' It was never in my consciousness, even. You could see yourself playing Lear maybe, down the road. But when you start with the robust silhouette of a man like Churchill, with the big jowls and the double chin, it's hard to see that."

He turned that project down, but was intrigued when another film came up two years later, mainly because it explores a specific month-long period in Churchill's life. "The script, I thought, took an interesting approach," Oldman says. "I wasn't being asked to play a life - Churchill through many, many years. It honed it and laid the themes into clothes I could try on. I could see how it would work. But the physical aspect was always going to be a challenge. I would have had to put on about 80 pounds, which I couldn't do. Makeup was the only answer."

Watch the trailer for 'Darkest Hour' here:

Once he realised that it could be done, he became fascinated by Churchill, reading his books and digging into this pivotal period in history. But inhabiting the great man was a challenge, and he decided from the start not to exaggerate specific mannerisms. "As an actor, you look at different aspects of a character," Oldman says. "I try to completely surround myself with the assignment. It's like being in a big cloud and then some of it rains through! For instance, looking at not only Churchill's way of walking and mannerisms and the way he sounds, but also looking into the psychology."

And because he was first to arrive on set to get the makeup in place, most of the cast and crew only ever saw him as Churchill. "Yeah, Joe actually didn't see me as Gary for three months," he laughs. "I would come out of the dressing room having already been through makeup. Actors playing soldiers would suddenly stand upright and come to attention. Ladies would curtsy. People would stare at me because the makeup was so good that you could literally stand an inch from me and you couldn't tell I was wearing any. It was fascinating to people. It was really like being the Prime Minister for a few months!"