He, Jo Jung Suk, and Park Shin Hye had an interview with 'SectionTV' for their upcoming movie. During the interview, Jo Jung Suk said, "The character curses a lot, but I'm not very good at cursing. But all of a sudden, I got really good at it. It's all the results of practice."
D.O added, "I got lessons from Jo Jung Suk-hyung on cursing. I learned pronunciation and attitude, and all of a sudden I got really good, too."
If you want to see Jo Jung Suk and D.O cursing, make sure to check out their movie 'Hyung'!