Actor Jung Yu-mi said it was director Bong Joon-ho who recommended she join the cast of “Sleep,” a mystery thriller by rookie director Jason Yu. Yu worked with Bong as an assistant for “Okja" (2017).

“Director Bong said a person he knows well wrote a scenario and that he hoped I would take a look at it,” Jung told The Korea Herald in an interview in Seoul on Aug. 22.

She appreciated the succinct and straightforward story. "There were no unnecessary things attached to the story, which, as an actor, I like," Jung said.

The 40-year-old actor, who starred in hit dramas and films including "Train to Busan” (2016) and “Kim Ji-young, Born 1982” (2019), said that a film's scenario and the chemistry with the director are priorities when it comes to choosing a project.

“I wanted to know more about the director after reading this very compact scenario. The director is always important to me. I wanted to hear from him how he planned to fill in the empty spaces and gaps in this film," Jung said, adding, "After meeting him, I had faith in him and decided to do this project.”

In “Sleep,” Jung plays Soo-jin, a newly married pregnant wife who is worried about the strange sleeping habits of her husband, Hyun-soo (Lee Sun-kyun). Hyun-soos's bizarre nocturnal behavior changes Soo-jin's attitude toward him.

It’s not Jung's first time playing a character driven to madness. In “Psychokinesis” (2018) she plays a ruthless real estate developer who orders the razing of buildings for a redevelopment project, with power and money in her hands.

“To be honest, I didn’t realize until quite recently that my role in this movie was that of a woman slowly descending into insanity. I thought Soo-jin was just a wife who wanted to help her husband," Jung said. Now, Jung noted that she regrets not having leaned into the role more.

In response to a comment that “Sleep” shows a "different side" to her acting, Jung said she appreciates director Yu's guidance.

“I don’t personally enjoy checking my performance on the monitor on the set, unless the director asks me to come and check. I was surprised to see myself on the screen in ‘Sleep,’ because I could see how the director tried to show different sides of me," she said.

Jung added that it was her trust in the director that allowed her to feel a sense of freedom when she was acting.

“I love it when directors are very clear about their direction and say things in a technical way, telling me exactly what to do. For this project, I just did what I was told. In that way, I felt very very free, enjoying every moment and feeling comfortable with my acting,” she added.

“Sleep” hits local theaters on Sept. 6.