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The upcoming movie Operation Mekong is the first cinematic adaptation of the Mekong massacre, which occurred in the Golden Triangle area in 2011.[Photo provided to China Daily]

Lam tells China Daily that the ministry allowed him to read confidential material about the attack before he started work on the film.

"I'm a Hong Kong filmmaker. So, I knew very little about Chinese mainland culture and its legal systems. I feel very lucky to have been able to direct this film.

"I spent a lot of time on research because the movie needs a good balance between reality and commercial compulsions," says the 51-year-old director.

In a career spanning nearly three decades, Lam is known for his award-winning cop vs gangster crime thrillers, a key genre in Hong Kong cinema's golden era.

He is also very adept at bringing in the human angle to dramas boasting fierce gunfights and bomb explosions.

An old-school filmmaker who insists on real-life settings for his films, he made the crew travel along the route taken by the vessels.

Recalling the shoot, Zhang, who plays a Chinese police officer in the film, says: "We once filmed in a river in the Golden Triangle area. As each ship was at least 1 kilometer away from the other, the director used a motorboat to shuttle between the two vessels during the shoot.

"He is so incredibly energetic."

The cast says that they had special training in close combat and using guns in Singapore, Thailand and Malaysia.

Peng, who starred in Lam's To the Fore (2015) and Unbeatable (2013), says: "From the sky and deep waters to caves in remote mountains, the sets were beyond imagination."

"Lam is always so demanding," says the Canada-educated Peng.

Zhang, who nursed similar feelings, says: "I once thought we would use computer-generated image technology to shoot some action sequences. But Lam made me wear a 20-kilogram vest, hold a heavy a gun, and run faster than a patrol dog."

Meanwhile, despite all the complaints, the cast says that their efforts and the visual spectacles will bring in the viewers.