But the director, best known internationally for "Oldboy" and "Thirst," which each won Cannes festival prizes, discovered unexpected benefits.
"It was a new experience compared with making a meticulously planned movie. Even a casual and spontaneous shot delivered a surprise," he said.
"It felt like there were more choices."
The scenes were shot simultaneously with two iPhones from different angles, but staffers also contributed with recordings on their own iPhones.
"Some of them had an unexpectedly interesting angle," said Park Chan-Wook, describing the process as more democratic since everyone with a smartphone took part.
PROne, the agency representing Park Chan-Wook, claimed the iPhone movie would be the first ever to be shown in cinemas.
Park Chan-Wook, however, said the medium would not outweigh the message.
"Making a film with smartphones might generate more interest at the moment. But as time goes by, stories and actors on screen will be seen as more important," he told Yonhap.
KT Corp, sole local agent for the iPhone which has sold 1.8 million units in South Korea since November 2009, partially funded the film.