Sally Field had to go to bat for herself to get the role of Mary Todd Lincoln in the upcoming biopic Lincoln.

Director Stephen Spielberg, who had been attempting to make the film for more than a decade, first spoke to Field about playing the wife of the 16th U.S. president in 2005.

But when the actor originally cast as President Abe Lincoln dropped out and Daniel Day Lewis stepped in, Field knew it would be the role of a lifetime.

It was Day Lewis, an actor Field calls “a towering example of uncompromising excellence” who she had to convince.

In an interview with CBC’s Q cultural affairs show, Field says Day Lewis flew in from Ireland to meet her after she did her screen test. They met in character – him as Lincoln, Field as Mary – and did improv in the roles without a script for about an hour before she was formally offered the role.

Field put on 25 pounds to play Lincoln’s wife – a process that took her about seven months – and read everything she could find on the Kentucky-born belle who supported her husband both socially and politically.

Field said she needed to “put together the psychological ingredients to make this person behave as it was documented she had behaved and to bring her alive on the interior and exterior.”

In an interview with CBC's Deana Sumanac, she explained Mary's role in advancing her husband's career and what the film Lincoln says about democracy.

Field has won two Oscars, for Places in the Heart in 1984 and Norma Rae in 1980, and is being tipped as a nominee again for her role in Lincoln.

The actress, who began her career in TV sitcoms, said she often had to fight early in her career for serious roles, including the role of Sybil, a woman with a split personality, which catapulted her into film and more serious drama.

Lincoln is released this Friday in Toronto, Montreal and Vancouver.