Gong Yoo plays Ki-heon, a character who takes narcotic painkillers to overcome constant pain and seizures while living a low life. 

Gong Yoo tried to lose weight and maintain that weight for about 4 months to express the “heavily damaged” appearance that cannot be portrayed through makeup. 

Gong Yoo

Even during the filming period, he couldn’t eat and drink together with the team on the set. Due to this, he didn’t have time to hang out with the staff, which led to him unintentionally becoming an outcast…

Source: Daum