In order to get in character, JANG Dong-gun underwent intense training physically and mentally. He trained with a U.S. special operations unit so that he could move like the killer Gon is and even read essays by real adoptees to truly understand his character's emotions and trauma.
The tattoo on the left shoulders of Gon and Chaoz is the emblem for the legendary Vietnam War unit MAC-V SOG. Although they were disbanded in 1972, the movie may be implying the backstory between them as being former U.S. Special Ops. And the tattoos were a continuity error.
In Spain the movie was released directly on VOD (digital platforms) and Blu-ray/DVD, but was previously a technical release in Barcelona (Phenomena) for 1 day in subtitled version.