The romantic film, Paris Holiday, will be screen in theatres in July.
Louis Koo had to take on a scene very different from his usual one: dancing the waltz.
According to Hktopten, two dance instructors were hired to give them a quick dance lesson.
Louis and his co-star, Amber Kuo, reportedly practised for over three hours.
Louis was reminded to look into his partner's eyes when he danced. However, later when he and Amber practiced, he kept staring at Amber and this instead made both of them laugh.
They gradually picked up the dance moves and even smoothly completed the over three-minute-long choreography on one try.
Both instructors praised them for their determination and chemistry.