Hyun-Jung Go (Middle aged Momi) is actually 5 years older than Yeom Hye-ran (Oh-nam's mom Kyung-ja).
Lee Han-Byul was selected to play Kim Mo-Mi after participating in an audition with around 1,000 other actresses. "Mask Girl" is her first acting role
Based on webcomic "Mask Girl" written by Mae-mi and illustrated by Hee-se (published from August 15, 2015 to June 9, 2018 via Naver.).
During an interview, director Kim Yong Hoon explained why he cast different actors (Lee Han Byeol, Jin-Ah Im/Nana, and Go Hyun Jung) to play the same Kim Momi role. "After committing murder, Momi would naturally be different. And then, after spending years in prison, she feels like another Momi altogether. I wanted to express her differently from the Momi we've seen earlier on in the story." The show's dance sequences provided unique challenges and surprises. Speaking of Lee Han Byeol's dance, Kim said, "Momi had to be a good dancer as per the script. Lee Han Byeol couldn't perfectly execute the dance, so I had to think of a dance double who could nail it." He laughed, recounting how Jin-Ah Im'd dance vibe was so unique, her being a K-pop idol previously, that he had to re-shoot it. "(The opening dance) was a blend of Jin-Ah Im, the dance double, and Lee Han Byeol's leg splitting. I mixed it appropriately."
Actress Jin-Ah Im is also known as singer Nana of the K-pop girl group After School.