Compare Lincoln's Gettysburg Address to letters by Lincoln.

Lincoln's Gettysburg Address is a masterpiece of hope, graceful writing, and brevity. His letters at times show the same mastery of style. For example, in 1846, Lincoln wrote to Allen N. Ford, the editor of the Illinois Gazette, refuting charges Lincoln's opponent a congressional campaign that Lincoln scoffed at religion. Lincoln wrote of his opponent, "I have little doubt now, that to make the same charge -- to slyly sow the seed in select spots -- was the chief object of his mission." This excerpt shows his ability, also expressed in the Gettysburg Address, to make a strong claim and to support it with language that is both concise and beautiful. Lincoln's use of alliteration ("slyly sow the seed in select spots") in this letter mirrors the alliteration he used at the beginning of the Gettysburg Address: "Four score and seven years ago our fathers brought forth on this continent, a new nation" ("score" and "seven" and "new" and "nation" are alliterative).
Lincoln's letters show a sense of humor that he did not display in formal speeches such as the Gettysburg Address. For example, in his 1860 letter to 11-year-old Grace Bedell, who suggested that Lincoln grow a beard to improve his chances of being elected president, Lincoln wrote, "As to the whiskers, having never worn any, do you not think people would call it a piece of silly affection if I were to begin it now?" This letter shows a wry and personal tone that Lincoln did not use in his speeches. Later, as president, he visited Grace Bedell while sporting a beard.
Lincoln also showed more emotionality in his letters than in his speeches. For example, he wrote to the Ellsworths in 1861 upon the death of their son, who was a colonel in the Union Army. Lincoln not only expressed his condolences to the family but also showed his own sense of connection with the younger man when he wrote, "My acquaintance with him began less than two years ago; yet through the latter half of the intervening period, it was as intimate as the disparity of our ages, and my engrossing engagements, would permit." Lincoln displayed a sense of emotion that he did not show in his speeches. In both his private letters and his speeches, Lincoln was a graceful writer who managed to express deep sentiments and hopes for the future with a rare sense of concision. In this, he is rare among presidents. 
http://www.abrahamlincolnonline.org/lincoln/speeches/letters.htm