Marks the twelfth time director Johnnie To and actor Louis Koo collaborate in a director/actor relation.
The Chinese title "San Ren Xing" comes of the first half of one of Confucious' analects. San Ren Xing means "three people walking". The full analect reads: "If three people walk together, at least one of them can be my teacher".
It means when in the company of others, even in a very small group of three, there is always opportunity to learn from them.
The slow-motion long-take shootout in the hospital ward administration station area, is not done by slowing down film shot at high speeds. It is shot at normal speed with the actors slowing down their movements and expressions to simulate slow-mo. It involves more than 100 actors practising for 2 months, and 4 days of camera rehearsal and positioning. A physical instructor and a language instructor were arranged to teach the actors how to control the speed and balance of movements. During the physical training, the actors had to learn slow-motion movements such as escaping, shooting, and turning. The camera goes around the room and captures the smallest of bloody details in all angles in one continuous (and digitally-assisted) shot.
In an interview, Louis Koo laughed and said that he thought it was a post-production slow motion at first, saying he knew there would be a slow motion scene before filming, and he thought it was a filming technique at first, but later found out that he had to do the slow motion himself.