According to the LA Times, Kushner (“Angels In America”) wrestled with the script for five years. The “Munich” screenwriter (arguably Spielberg’s best movie in the last decade) missed multiple deadlines as he took it from a 500-page, mini-series-length first draft to a 149-minute feature. He and Spielberg couldn’t tame it to their liking, so Liam Neeson moved on to work on other projects.
The original drafts seemed much more sprawling, going stem-to-stern through Lincoln’s life. The Times says the draft that came to Day-Lewis in 2003, the one he turned down, was "less presidential and more about the Civil War."
As you might imagine, Daniel Day-Lewis didn't break character between takes. “I just came to see him as the character. I assume he didn’t change the voice. Why would he?” Spielberg said.
The director and the actor were both concerned that actors speaking in their natural accents would distract Day-Lewis and inadvertently lapse into his normal accent so they were told only to speak to him with the American accents they had adopted for the role.
“It was sort of an extended improvisation,” Jared Harris, who plays Ulysses S. Grant in the movie, told the Times. “You didn’t go up to him and say, ‘Hey, did you see the Pirates game last night?’ It was important for him to retain the attitude, if you like, and the dialect he had created. So we would sit there and joke, for example, about the Vicksburg campaign.” He added, “At the end of the day sometimes we’d ride back in the car, and he’d stay in character but talk about ‘Mad Men,’ which of course he couldn’t know about, because television hadn’t been invented then.”
Interestingly enough, according to the New York Times, Kushner said that Day-Lewis warned him that once shooting began he would no longer be speaking to him and only to the director.