|Directed by||Kim Tae-kyun|
|Produced by||Kim Tae-kyun|
|Written by||Lee Seong-min|
|Music by||Kim Tae-seong
|Edited by||Choi Jae-geun
|Distributed by||Invent D|
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.
- 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
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 in her big screen debut.
The early working title was Strawberry Milk (Hangul: 딸기우유; RR: Ddal-gi U-yu), a drink which Jo's character has a fixation on. It was changed to Thorn because Kim said, "There is nothing sweet and soft about the movie like strawberry flavored milk. Only sharpness like a thorn exists. It represents something difficult."
- "Looking sharp". Korea JoongAng Daily. 12 March 2014. Retrieved 2014-05-06.
- Sunwoo, Carla (4 April 2014). "Emotions prickle, limits tested in Innocent Thing". Korea JoongAng Daily. Retrieved 2014-05-06.
- "Innocent Thing (2013)". The Chosun Ilbo. 11 April 2014. Retrieved 2014-05-06.
- Lee, Claire (7 April 2014). "Innocent Thing goes mad on screen". The Korea Herald. Retrieved 2014-05-06.
- Tae, Sang-joon (28 March 2014). "Innocent Thing: A Korean version of FATAL ATTRACTION". Korean Cinema Today. Retrieved 2014-05-06.
- "Jang Hyuk Talks About His Affinity for Melodramas". The Chosun Ilbo. 10 May 2014. Retrieved 2014-05-10.
- "Starlet Hits Right Note as Love-struck Student in Movie Debut". The Chosun Ilbo. 19 April 2014. Retrieved 2014-05-06.
- "Strawberry Milk to Thorn, the reason behind it...". Hancinema. 2 April 2014. Retrieved 2014-05-06.
- Lee, Cory (2 October 2013). "Jang Hyuk's New Romance Film Cranks In". TenAsia. Retrieved 2014-05-06.
- Tae, Sang-joon (21 April 2014). "Box Office: April 3-16, 2014". Korean Film Council. Retrieved 2014-05-06.
- Kim, Hee-eun (15 April 2014). "Captain America tops box office". Korea JoongAng Daily. Retrieved 2014-05-06.