Michelle Rodriguez played a third character. Apart from her good and evil versions she also portrayed a zombie doppelganger. As Ada Wong explains Umbrella tests every clone in different levels of Biohazard. The scene was cut in the editing process along with many scenes in the suburban house with Alice and her family.
During the car chase with the Zombie Russian army the music playing is called 'Phantom Chase' by tomandandy. This references music from the original Resident Evil (1996) video game, particularly 'Second Floor Mansion'.
Despite the fact that it is one of the first scenes in the film, Alice's fall in the water was one of the last sequences to be filmed. There was an extra shot of Jill Valentine and Umbrella soldiers pulling Alice out of the water which was cut in the edition process.
Las Plagas is a parasite in the video games. However its depiction in the film is not accurate, since in the games it is merely a symbiotic entity and not a microorganism which grants immunity to injuries as in this film.
While filming a fight scene with Milla Jovovich, a mishap occurred causing 16 actors to fall off a scaffolding and sent to the hospital. As a result, filming was shutdown for a few days and Milla Jovovich visited some of the actors in the hospital out of respect.
Every other Resident Evil film had a different composer for the soundtrack. This is however the first time that the music was written by the same people in two consecutive films. Tomandandy (a duo of composers) scored Resident Evil: Afterlife (2010) and returned to write the soundtrack for this film as well.
There are 3 scenes from Resident Evil: Afterlife (2010) that were reused in this movie: 1. J-pop girl attacking Japanese businessman 2. The scene showing A serum flowing through Alice's veins 3. The Ospreys approaching the Arcadia to attack.
The original plan of writer/director Paul W.S. Anderson was to film this movie simultaneously with the upcoming sixth installment in the series, making essentially a two part finale. In the early stages of development Ali Larter and Wentworth Miller were slated to be included in the cast list. When the studio backed out of the concerted filming for two movies, Claire and Chris Redfield were scrapped from the production plans and the filmmakers made it clear that Resident Evil Retribution would be a standalone film. The only carryover from the initial conception is the tagline which bills the 5th film as the beginning of the end.
The scene between Ada Wong and Alice in the underground office is a re-enactment of the scene in Resident Evil 4 video game where Leon S. Kennedy meets with Ada for the first time in the Salazar Family castle bedroom.
The idea of cloning the umbrella soldiers may have stemmed from a comment made in an audio commentary of the original Resident Evil (2002) film. 'Michelle Rodriguez' said that 'Colin Salmon' should be brought back in a sequel as a clone. Ultimately, this came true for Salmon as well as Rodriguez and 'Oded Fehr'.
This is the second Resident Evil spin-off to feature Leon Scott Kennedy and Barry Burton working together in an "anti-umbrella" unit. The two also collaborated in the non-canon _Resident Evil: Gaiden_. Although, the story reveals that the Leon in that game was an impostor and the real Leon had died.
Alice and Becky and especially the scene which Alice frees Becky from a cocoon was written as a obvious nod to "Aliens", mostly the characters of Ripley and Newt and the scene which Ripley frees Newt from a cocoon in the confrontation with the Alien Queen.
The trivia items below may give away important plot points.
The ending of the 4th film was shot on the exterior of a real abandoned ship. When it came time to film the opening sequence of this film however, the crew were surprised to discover that the ship had been back in use and therefore was no longer available. As a result the Arcadia battle was eventually filmed on a soundstage replica of the real ship.
Although not mentioned in the finished film the back-story reveals that the majority of the Umbrella soldiers are clones. This accounts for their vast numbers and explains their allegiance to Umbrella. The only time that this is hinted at is during the scene after the reboot in the facility systems. Jill Valentine is activated through the Red Queen and all the soldiers seem to wake up simultaneously.
According to the official novelization Alice intended to dock the Arcadia ship in the nearest island, Katalina. She believed it would be a safe place with few undead creatures. The trip was never realized because of the attack. The novelization clarifies that during the gunfire Alice lost the Redfields from her eyesight and never knew what happened to them.