Director Benny Chan described the filming process as some of the darkest days of his career, explaining that the baby was a factor beyond his control, and could not work more than eight hours a day. Whenever a scene involving the baby was shot, the crew members had to be silent, communicating in sign language. Benny Chan stated it took time, patience, and money to guide the baby through each shot, and numerous retakes were required due to the baby's constant crying and napping. As a result, production went over budget. Special methods were sometimes used to coax the baby into co-operation. One scene required the baby to suckle Chan's nipple. The baby was initially hesitant, but relented after the crew brushed a large amount of honey onto the nipple.
Jackie Chan sings the theme song for this movie which is evident in the outtakes. Another version is also out with Michael Hui and Louis Koo singing along with Jackie.
Daniel Wu and Nicholas Tse, stars of New Police Story (2004), make cameo appearances as gay security van drivers during a car chase in this movie.
Although Louis Koo co-starred with Jackie Chan, several action scenes involving his character were shot with a stunt double. In addition, Koo was the baby's favorite on-set. Whenever the baby cried, Koo was always the first to cheer him up.
The producers had originally intended Michael Hui to fight along with Chan and Koo. However, it was eventually decided that Hui would simply act as the brains of the gang.
Jackie Chan had originally intended to co-star with Biao Yuen and Sammo Kam-Bo Hung, reuniting the trio for the first time since Dragons Forever (1988). However, Hung declined due to scheduling conflicts.
Over one hundred auditions were held before the suitable baby was found to appear in this movie. Benny Chan chose Matthew Medvedev, a one-year-old infant of Chinese and Colombian descent. Medvedev, known as "Baby Matthew", was literally recruited off the street when an assistant director spotted him with his parents on the MTR. Although his family was simply visiting Hong Kong, they agreed to stay and let Matthew appear in this movie.
Jackie Chan suffered minor injuries attempting stunts in this movie, having been kicked in the chest by a stuntman wearing the wrong boots and fallen off a quad bike, while attempting to perform a wheelie.
The Japanese title of this movie is "Purojekuto BB", literally "Project BB", the name initially proposed for the movie.