The Irish actor is famous for his fixed method acting and he remained in character throughout the shoot, even asking the cast and crew on Steven Spielberg's period movie to address him as Mr. President on set.
Gordon-Levitt landed the role as Lincoln's son Robert after Day-Lewis personally asked for The Inception star to be cast, and he received a special note from the veteran once the deal was signed and sealed.
Gordon-Levitt says, ""He gave me a letter, a handwritten letter written in old-fashioned fountain pen on old-fashioned stationery. And from what I understand, he actually studied Abraham Lincoln's penmanship and he wrote it sort of as himself, sort of as Lincoln and he signed it A/D, so Abraham and Daniel.
""It was amazing. He was just welcoming me to the job and I think he wanted to extend a warm welcome because then when I met him personally... it was, 'Hello Robert', and not very warm because their (Lincoln and his son's) relationship is not very warm, and I said, 'Hello, sir'.""
Gordon-Levitt wasn't the only castmember to receive communications from Day-Lewis as the President - Sally Field, who portrays his onscreen wife Mary Todd, exchanged text messages with her co-star as the historical couple for months before cameras began rolling.