Mads Mikkelsen took on the lead role, thinking that it would be quite an easy gig for him, seeing as he had no dialog. He didn't realize the arduous physical demands of shooting on forebidding Scottish locations.
The genesis of the film comes from the discovery of a mysterious pile of runes near Delaware. There was been much speculation as to how they got there so far inland so the film seeks to provide an answer of some sort to that.
In writing the script Nicolas Winding Refn was responsible for the spiritual, metaphysical and plot related part while Roy Jacobsen was responsible for all the Norse mythological, viking and historical themes due to his extensive knowledge about vikings and Norse paganism.
Although Refn playfully refuses to admit on what basis One-Eye wears tattoos other than that "he just knows it", the famous account of the Rus - almost universally considered by scholars to be Swedish Vikings - by Ahmad ibn Fadlan says that the Rus wore tattoos similar to the ones of One-Eye.
The trivia items below may give away important plot points.
The film was supposed to end with a huge battle scene, with One-Eye fighting an entire army of Indians. Nicolas Winding Refn was given a budget for the scene and he started to plan the battle, before deciding that One-Eye should sacrifice himself so that The Boy might live.