The scenes involving Jet Li fighting himself were shot using Li and a stuntman with a build similar to his. The stuntman's face was painted green so that it could be digitally replaced with Li's face later.
For the martial arts aficionados: Jet Li uses two distinct kung fu styles when portraying Gabe and Yulaw. The evil Yulaw uses Xingyi Quan, or "Mind-Form Fist" (loosely translated). As can be seen when he is training, it is a very linear, offensive style. Gabe on the other hand practices Bagua Zhang, or the "Eight Trigrams Palm". It is characterized by open palm fighting, with circular footwork. Both styles are two of the three main "internal" styles of Chinese kung fu - the third being Taiji Quan (Tai Chi).
Each of the universes that are listed along with Yulaw's charges is named after a star or constellation (in keeping with the idea that a new universe is formed when a star collapses): Monoceros, Tucana, Canopus, Serpens, and Shaula. The one exception is Lawless's universe. His, along with the other universes that are actually shown in the film, are all named after mythological gods (a possible allusion to Yulaw's hope to become a god himself, by absorbing the energy of his other selves). Lawless is from Anubis universe, after the Egyptian god of mummification and the afterlife. Gabe is from Charis universe, after the Greek goddesses of charm and beauty. The penal colony is in Hades universe, after the Greek god of the Underworld. The colony itself is called Stygian Penal Colony: the word Stygian refers to the River Styx, which was located in the Greek Underworld. The location of the Multiverse Agency is called, simply, "Multiverse."
During the shootout at the beginning of the film, the LAPD officers in the Anubis Universe are all armed with M4A1 carbine assault rifles fitted with the M26 Modular Accessory Shotgun System. At the time of filming, the M26 was an experimental shotgun being developed for the United States Army by C-More Systems, and it was not available to the public. The M26s used in the film were prototype versions loaned to the production by C-More Systems, under a promotional agreement; they were later returned to the company after filming was completed.
As someone mentioned earlier, Yulaw uses xingyi quan. There is a scene in the movie where he is practicing his xingyi when he's shackled and jailed. This is a reference to the famous Xingyi master Kuo Yun Shen who did the same thing when he was jailed.
When Dwayne Johnson ('The Rock') was attached to star, this film was known as The Rock Movie. When he left to work on The Mummy Returns (2001), it was listed for a short while as Untitled Sci-Fi Project, and when Jet Li came aboard, it became The One.
When Yulaw first attacks Gabe in the L.A.P.D. Parking garage, Yulaw flees after reinforcements arrive. When they do Gabe yells, "HE'S GOING O.J.!" (i.e., "He's on the run!") This is a reference to O.J. Simpson's career as a running back in the NFL. (And a somewhat ironic quip due to Simpson's murder trial, considering that Yulaw had just escaped from his own.)
The scene where we first meet gabe's wife (Carla gugino) when she's getting ready to see if gabe is alright after being attacked, the background sound of the TV is heard playing the movie final destination (2000) one of james Wong's and glen Morgan's earlier films.
Filmed in 2001 but supposedly set well into Earth's future, in the first Earth-universe scene at the Earth-prison (about 20 minutes into the movie), just before Law (Jet Li) attacks the prison guards transferring a prisoner, a prison TV airs a news channel reporting this Earth's US Gov't is likely to enact "Universal Health Care" for its citizens . . . little did the movie's producers know that in just nine years, the Obama Administration would pass the Affordable Care Act.
There is constant mention that Yulaw killed 123 people. They are referring to his doppelgängers being killed. Oddly, there is never a count of 'collateral damage' (i.e., dead multiverse cops and civilians) which must number in the hundreds.