Who starred in South Korean film that "Pencil" was based on?
Pencil (film):  The film is based on South Korean film "4th Period Mystery" released in 2009. 4th Period Mystery: 4th Period Mystery (), released internationally as The Clue, is a 2009 South Korean thriller film starring Yoo Seung-ho and Kang So-ra.
Yoo Seung-ho and Kang So-ra