All of the U.S. flags in the film have 51 stars, because in the film's alternate history, Vietnam became the 51st state after America won the Vietnam War.
Zack Snyder found casting for The Comedian to be a difficult process, as the character is gruff and grizzled. Most of the actors he met with he felt were too groomed. Jeffrey Dean Morgan was very grumpy during his meeting with Snyder, something that appealed to him a great deal.
Jackie Earle Haley was the only one of the main cast who was already familiar with the graphic novel. He actively campaigned for the part of Rorschach.
Although Jackie Earle Haley has a black belt in Kenpo, he chose not to draw on those skills in his fight scenes, as he felt that Rorscharch would have fought in a scrappier, less disciplined style.
When offered the role of Adrian Veidt, Matthew Goode had never read the graphic novel. He called a friend who had and asked if he should even bother to read the script. Not only did the friend say yes, he insisted that Goode immediately read the graphic novel and accept the role without question. Later, after he read both the script and the novel, Goode admitted his friend was right to advise him take the role without delay.
Zack Snyder: (at around 45 mins) [director's & ultimate versions only] as a U.S. soldier during the Vietnam Battle. He is on the chopper with the Comedian.