The actress admits she was struggling with her role in the upcoming Steven Spielberg epic as she attempted to tackle Lincoln's wife during the months before filming started - and she even took to texting her leading man in character.
But it was a two-hour stroll around Richmond, Virginia - as Mary - that really gave her the confidence to pull the role off.
She tells WENN, "Daniel and I had not really spent any time together and I had to create a marriage with the man who you were intimate with on every single level of your being. Mary Todd had children with him and children died in her arms; she had a tactile relationship with this person and I had not spent any time with him, so we had no way of creating it.
"We had texted each other over seven months or something, totally in character, which was very difficult to do! I had to figure out what I wanted to say within the vernacular of the time and many times I wanted to call him up and say, 'How would I say this?'
"Right before we started shooting I arrived a week early in Richmond and Daniel was to start shooting the next day. I texted him as Mary: 'I knew that the task before you was an enormous one, but my task was you. And that was my sole task. And therefore I would be on your porch the next morning. Your choice is to let me in or not. Whether you let me in or not I will not go.' Mary would take him out for carriage rides so I texted, 'In lieu of a carriage, find your shoes.' And he wrote me back graciously.
"He's notorious for not really wanting to have any socialisation but I went to his house, knocked on his door in Richmond and we were both sort of in character. My dialect wasn't as thick as his. He said, 'Would you like to come in and have some coffee and eggs?' I said, 'I don't want to be trapped behind a table. Let's get going.'
"I knew he was under the gun in a very big way but we went out and we walked around Richmond for about two hours and it saved my life. I said, 'I have to create something that doesn't exist right now and first of all I have to touch you.' He said, 'OK,' so I laid my head on him. I said, 'I can't be worried and be shy about owning your body because married people don't have that,' and so his eyes got a little bigger and he said, 'OK'. So I would take his hand or grab his arm while we talked about our real lives and our children. At the end he invited me in but I said, 'I think we did enough and I think it served it's purpose'; and it did."
And she has nothing but great things to say about working with one of Hollywood's most serious actors: "He is so generous and such a deeply honourable, loving man. The first scene Daniel did was not Daniel anymore. I had no feeling of that. He was Mr. Lincoln."
In fact, Joseph Gordon Levitt, who plays Lincoln's son in the film, reveals the character never left Day Lewis throughout the shoot and he only got to meet the actor when he had wrapped his scenes - and introduced himself to his castmates.