No one knows what Abraham Lincoln looked like smiling. The protracted photography of his day made maintaining a smile while an image was being captured nearly impossible. So we know only his stern, contemplative expression, as it is memorialized in imposing marble on the Mall.

How to recreate the 16th president's unknown smile is just one among many challenges that actor Daniel Day-Lewis will face when playing Lincoln next year in Steven Spielberg's long-anticipated film based on Doris Kearns Goodwin's book, "Team of Rivals: The Political Genius of Abraham Lincoln."

Day-Lewis's casting was announced in late November, after Liam Neeson aged out of the part (Spielberg's plans for the biopic having been so long in the making). Day-Lewis is famous for his intensive method-acting technique, while filming the movie that shot him to stardom, "My Left Foot," for example, Day-Lewis insisted on remaining in a wheelchair even when the camera was not rolling, refusing to break from his paralyzed character. Day-Lewis will no doubt dedicate himself to "becoming" Lincoln. To get a sense of what this might entail, POLITICO sought out some experts.

Early on, Spielberg brought in renowned Lincoln historian Harold Holzer to offer some advice to Neeson when he was still signed on. He told POLITICO that, Lincoln's idiosyncrasies aside, portraying famous figures from the past poses perennial challenges.

"American audiences are difficult to please on historical drama. There's an inhibition about seeing characters in period costume that might not be as popularized, hearing speech patterns of earlier times and so on. Or, people who care about the history demand complete fidelity to the known record."

Holzer said that Lincoln was thought to have had some particular traits that are often lost in popular portrayals or caricatures of the president, and that a meticulous actor like Lewis might explore.

On that illusive smile, Holzer said Lincoln was "supposedly a big smiler, with brilliant white teeth that would have been rare for the time."

"When he told a story, it was almost childlike. Apparently he would turn from person to person to gauge their responses. And he was known to have hugged his knees to his chest in delight at his own stories. And to giggle when he laughed."