Spending much of his free time sipping coffee at cafes or hanging out with friends, Cheng recalls being captivated by the unique atmosphere of Shanghai, which led him to delve deeper into the city's history.

The historical chapter that fascinated him most was the city's complicated status during World War II.As it is depicted in the movie, Shanghai — then China's most cosmopolitan city — saw struggles and conflicts between Japanese invaders, its puppet government, resisting Kuomintang spies and the Communist Party of China's underground workers.

Although he has had a lengthy career as a filmmaker spanning more than two decades, Cheng has directed only four feature-length movies, making him known in Chinese movie circles for a unique style.

From his directorial debut Criminal (1999) to Lethal Hostage (2012) and the last outing The Wasted Times (2016), Cheng's films are mostly characterized by their art-film style, deftly structured suspenseful narratives, and nonlinear storytelling techniques. Through his works, the director explores the complexity of human nature and the impact of historical events on individuals.

So, despite having barely finished the script before the start of shooting, Cheng managed to attract some top stars to act in his films.

He recalls that Leung, one of the most sought-after Chinese actors, felt good while reading the script draft of Hidden Blade, which had just 30 pages, or a quarter of the regular length, after Cheng contacted his wife, actress Carina Lau Ka-ling.

After watching The Wasted Times in a Hong Kong cinema, Leung decided to collaborate with Cheng, traveling to Shanghai for preparations, including practicing his Mandarin lines, which was a challenge for the Cantonese-speaking actor.

Interestingly, Cheng, also served as the scriptwriter of all his movies, often revised the script or wrote new content for unfinished tales during the shoot, as he says he believes that the stress brought by the deadline gave him inspiration.

One example of the results of this unique "habit" is the character played by actor Huang Lei, who portrays a CPC underground worker that turns out to be a traitor.