Main •
Lee Jong-suk as Park Chang-ho : A third-rate lawyer with a ten percent success rate, called "Big Mouth" by his legal acquaintances due to his tendency to speak before acting. His life is suddenly in danger when he is mistaken for a genius conman known as "Big Mouse". •
Im Yoon-ah as Ko Mi-ho : Chang-ho's supportive wife who is a nurse and has a bold personality but succumbs to cancer. •
Kim Joo-hun as Choi Do-ha : The ambitious mayor of Gucheon, whose goal in life is to become the most dignified president.
Supporting NR Forum •
Yang Kyung-won as Gong Ji-hoon : The president of the media conglomerate Gukdong Daily and the chairman of the NR Forum. • Lee Yoo-joon as Han Jae-ho : A surgeon. • Oh Ryung as Lee Du-geun : Legal counsel of NR Forum. • Kim Kyu-seon as Ashley Kim : Ji-hoon's wife, who is a Korean-American and the director of Woojung Gallery. •
Jang Hyuk-jin as Choi Jung-rak : A prosecutor at Gucheon Public Prosecutors' Office. •
Park Se-hyun as Jang Hee-joo : A nurse at Gucheon Hospital. • Kim Dong-won as Gan Su-cheol : A prison officer at Gucheon Prison. •
Kwak Dong-yeon as Jerry / Oh Jin-chul (real name) : A scammer with three previous convictions who respects the genius conman "Big Mouse". • Yang Hyung-wook as Noh Park : A prisoner at Gucheon Prison.
People around Chang-ho •
Lee Ki-young as Ko Gi-kwang : Mi-ho's father.
Extended • Yoo Tae-ju as Tak Kwang-yeon : A psychopath death row inmate at Gucheon Prison. • Song Kyung-cheol as Yang Chun-sik : A gang leader. • Park Jeong-bok as Go Tae-sik : A prisoner at Gucheon Prison. •
Shin Seung-hwan as Peter Hong • Jeon Gook-hwan as Kang Seong-geun (Chairman Kang)
Special appearances •
Yoo Su-bin as Chang-ho and Mi-ho's neighbor. •
Kim Do-wan as Chang-ho and Mi-ho's neighbor. ==Production and release==