Innocent Thing (; lit. "Thorn") is a 2014 South Korean romantic thriller film directed by Kim Tae-kyun, starring Jang Hyuk and Jo Bo-ah. Plot Joon-ki (Jang Hyuk) was once a former rugby athlete and is currently a popular physical education teacher at an all-girls' high school. His pregnant wife Seo-yeon (Sunwoo Sun) is due to give birth to their first child imminently. Although he's used to playful advances from pubescent students, things take a dangerous turn when Young-eun (Jo Bo-ah) falls hard for her teacher and daringly confesses her feelings for him. Amid growing rumors, he is soon consumed with guilt and attempts to end the relationship. However Young-eun's pure crush slowly turns to obsession. She starts to regard everyone related with Joon-ki as obstacles, and the farther he keeps her away, the bolder her obsession and madness grows. Cast Jang Hyuk as Kim Joon-ki Jo Bo-ah as Ha Young-eun Sunwoo Sun as Seo-yeon Lee Do-ah as Min-joo Do Gwang-won as Seok-jin Jang Seo-kyeong as Joo-hee Shin Cheol-jin as school custodian Park Jin-young as Jang-in, Joon-ki's father-in-law Do Yong-gu as gynecologist Wang Tae-eon as principal Kim Kyeong-sik as Teacher Park Han Hee-jung as housekeeper Son Hee-seon as Joon-ki's mother-in-law Yoo Ji-yeon as Cheo-hyeong, Joon-ki's sister-in-law Kim Won-sik as Dong-seo Yoon Bok-in as OB/Gyn nurse Baek Eun-joo as student council president Kim Ye-eun as Ye-eun Han Seul-gi as Seul-gi Lee Sang-mi as Sang-mi Park Jeong-hee as Jeong-hee Lee Sae-bom as Sae-bom Woo Do-im as Do-im Park Gyu-ri as Gyu-ri Kang Do-eun as Joo-hee's mother Jeong Joon-yeong as Teacher Jeong Choi Won-seok as Won-seok Heo Ji-hye as new ward nurse Kim Mi-kyeong as neonatal nurse Oh Ui-taek as male student Lee Yeon-joon as "kiss" man Park Joon-seo as man in TV drama Kim Bo-ryeong as woman in TV drama Production The film reunited director Kim Tae-kyun and actor Jang Hyuk for the first time in thirteen years since Volcano High (2001). Kim first encountered the screenplay in 2007 at a competition, and he was won over right away, captivated by the blurred lines between love and obsession. He called the film "an experiment" due to its eclectic mix of suspense, horror, melodrama and romance, and that his use of cinematography and camera techniques were essential in delivering the "feeling of being trapped." Wanting the schoolgirl character to embody both innocence and pure sex appeal, Kim auditioned 250 actresses, before casting Jo Bo-ah