Unbeatable director Dante Lam and actor Nick Cheung are fast becoming an unstoppable team as they unleash their new movie That Demon Within.

Three cops engaged in a gun battle in a subway. Two are found dead while one is seriously injured. This is the story of the rogue cop that rocked Hong Kong in 2006. And it is the springboard for Hong Kong director Dante Lam’s latest action thriller That Demon Within.

In a recent phone interview from Hong Kong, Lam shared that his new movie is not a story exactly based on Hong Kong’s notorious ‘Demon Cop’.

That Demon Within may be inspired by those events, but the plot for the movie is entirely fiction, save for the infamous gory subway scene. It is one which Hong Kong people will know too well,” said Lam, 48.

The thriller stars Daniel Wu and Nick Cheung; the latter recently grabbed headlines after winning the Best Actor trophy at the 2014 Hong Kong Film Awards for Unbeatable – another movie directed by Lam. Cheung’s collaboration with Lam has proven to be most fruitful; the 2008 Beast Stalker won him seven acting accolades and propelled him to superstardom.

Lam recalled how he had already gotten both Wu and Cheung on board, but had yet to decide on the final casting. After doing extensive research and consultation with psychology experts, Lam made up his mind.

“After examining our storyline and the psychological condition of our protagonist, it was decided that the character had to be younger, which explains how he could still be so deeply affected by childhood trauma,” the director explained.

Since Wu is 39 while Cheung is 46, the former was then cast as the reclusive cop and the latter, the ruthless criminal.

In That Demon Within, Wu plays strait-laced policeman Dave Wong, who unknowingly saves the life of the vicious psychopath Hon Kong, then becomes obsessed with apprehending the ringleader of the Demon King gang of robbers,

Though Cheung may have been accorded less screen time this time around, work was equally daunting. “It was mentally exhausting this time, as I had to create a diabolical persona for this character, sometimes with just a smile, as he doesn’t say too much.”

And even though Cheung did not have to spend months conditioning his body like he did for Unbeatable, the physical challenges were no less demanding. The perfectionist in him meant he constantly insisted on performing all his own stunts and refused to use body double even in long shots, where his character was wearing a full-face mask.

The multiple-award-winning actor also had to endure hours of makeup for several of his scenes. One of the most extensive sessions required him to endure some five hours of having prosthetic makeup applied on his face for a fiery inferno scene. “But, the removal of prosthetic makeup adhesive is even worse as the process took hours and requires the continuous application of special solvent that made my face stiff and red,” he recalled.

After being in front of the camera for 25 years, Cheung made his directorial debut with a horror flick that was filmed in Malaysia last year. The movie which tells of spooky events encountered by a Cantonese opera troupe during the Hungry Ghost Festival is due to hit cinemas later this year.

> That Demon Within opens in cinemas nationwide today.