The unique cameo came about thanks to the fact that the director and staff of 'Dear My Room' also previously worked on '100 Days My Prince'. Furthermore, Nam Ji Hyun's older brother from '100 Days My Prince', actor Kim Jae Young, plays the male lead of 'Dear My Room'!
In their cameo, D.O. and Nam Ji Hyun transformed into a present-day, married couple, appearing as interior design clients of Kim Jae Young's character Seo Min Suk. Nam Ji Hyun then gives romantic advice to her former older brother by telling the story of how she married her childhood friend, D.O.
Later, D.O. arrives to pick his wife up from the meeting, and comments, "But... am I the only one who feels uncomfortable about that person? I don't know... I feel like I've seen him somewhere before..."
In their making film (above), you can also catch a glimpse of actress Lee Min Ji, who played Nam Ji Hyun's best friend in '100 Days My Prince', as well! You can catch D.O. and Nam Ji Hyun's cameo appearance in the clips below.