On the movie's second day of release, June 6, 2013, "Secretly Greatly" broke the all-time single day ticket sales record for a Korean film. "Secretly Greatly" sold a total of 919,036 tickets on June 6, 2013, easily surpassing "The Thieves," which previously held the one day record with 759,590 admissions. U.S. film "Transformers 3" holds the all-time single day record with 956,500 admissions.
Filming began October 17, 2012 and ended March 8, 2013. Shooting took 5 months to complete. The last scene filmed took place in an alley in a low income neighborhood. The scene involved Dong-Goo (Kim Soo-Hyun) and Hae-Jin (Lee Hyun-Woo) watching something secretly.
Based on webcomic "Covertness" by Hun (birthname Choi Jong-Hoon). "Covertness" was first published from June 30, 2010 to May 4, 2011 on Daum website.
At 36:08 An anti-materiel rifle is a rifle that is designed for use against military equipment, rather than against other combatants.