According to Travis Beacham, an earlier version of the script would have Mako (Rinko Kikuchi) and Raleigh (Charlie Hunnam) speaking two different languages for the majority of the film. After connecting as pilots, they would slowly begin to understand one another, and before the end would hear each other speaking in their own respective languages.
When Little Mako is in the alleyway, every object on the set was rigged to the same hydraulic system. Whenever the giant monster was to take a step, everything bounced or shook in unison, including the puddles.
A life-sized version of the robot cockpit was built on a soundstage at Pinewood Studios in Toronto. It weighed about 20 tons, and stood nearly four stories high. It was built on a gigantic hydraulic gimbal, which would move, shake, vibrate, drop, and rock the entire set as if it were actually being piloted. A smaller version was also built with a smaller gimbal, allowing for different movements. The VFX team used some of the Conn-pod footage for reference while animating the robots. The set was also redressed to depict the interior of each robot differently.
The computer is voiced by Ellen McLain who also voiced GLaDOS, the AI from Portal (2007) and Portal 2 (2011). This is in fact a cameo by GLaDOS, as Guillermo del Toro was such a fan of the games that he approached the game's developers, Valve, who approved. Del Toro said in an interview for the Toronto Sun: ""I wanted very much to have her, because I'm a big Portal fan. But just as a wink. She's not cake-obsessed. She's not out to destroy humanity." He further explained: "Look, there's no A.I. I'd rather have than GlaDOS, but McLain's voice in the movie, due in theaters July 12, has been modulated a bit to be less similar to the distinctive tone of Portal's unforgettable antagonist. The filter we're using is slightly less GLaDOS. Slightly. The one in the trailer I wanted to be full-on GlaDOS." The GLaDOS voice itself is inspired by the computer in The Thirteenth Floor (1999).
Screenwriter Travis Beacham also wrote the graphic novel Pacific Rim: Tales from Year Zero. Released along with the movie, Tales from Year Zero serves as a prologue to the film and is set twelve years before its events.
Travis Beacham, the screenwriter, got the idea for the movie while walking along the California coastline on a foggy morning. The shape of the pier looked like a creature rising from the water, and he imagined a large robot waiting on the shore to battle it.
Visual effects supervisor John Knoll and Guillermo del Toro spent several weeks discussing the physics of the giant characters, and went into very specific detail. Such as how the air displacement from a Jaeger moving between skyscrapers would shake the building's windows.
In March 2013, YouTube channel "Dumb Drum" created a "sweded" version of Pacific Rim's trailer. Less than two weeks later, Guillermo del Toro commented on the video at WonderCon 2013, calling it a "masterpiece" and saying that it inspired him to create the second trailer for the film. He also invited the filmmakers to the Hollywood premiere.
The Kaiju's voices are comprised of layer upon layer of animal roars and growls which were filtered, sped up, and slowed down to create the roar of alien behemoths. Then, to add emotion and a sense of intelligence, supervising sound editor Scott Martin Gershin and director Guillermo del Toro added samples of their own voices.
Karloff, one of the kaiju from the opening sequence, is named after Boris Karloff. The nickname was given because the creature's head resembles the dessicated face of Karloff's The Mummy (1932) character, Imhotep.
This was both director Guillermo del Toro and cinematographer Guillermo Navarro's first experience shooting with digital cameras. Navarro bought seven Red Epic cameras specifically for the film and used his own lenses, as he prefers shooting with his own camera equipment over renting it.
In Hungary, the trailers for the movie could not mention the name of the main robot, Gipsy Danger. The reason being that the name was seen as offensive against gypsy people, who form a large ethnic group in the country. In the dub of the movie itself, the name is spoken out freely, but it's left in English. This is because it is a reference to a type of airplane engine, not the people in question.
When the Gipsy Danger is fighting the second Kaiju in Hong Kong, it punches through a building and sets off a set of Newton's Balls or Newton's Cradle. The scientist that the Kaiju was chasing is named Dr. Newton Geizler.
Pacific Rim's Jaegers are a staple of Japanese anime, where they are often referred to as mecha. Similar to many mecha anime series, Jaegers are controlled from within by human pilots, distinguishing them from other depictions of robots as automated, sentient, or externally controlled.
The inside of the building that Gipsy's fist smashes through was created using miniatures. Many of the components of the office cubicles were made using 3D printers. The lighting was practical, and done to scale. Once the set was ready, a large green mandrel was rammed through it. This was digitally replaced with Gipsy's fist.
In the weeks before the film's release, several popular YouTube channels collaborated with the filmmakers to produce a short video titled "Pacific Rim: Training Day." The short even features a brief cameo by Guillermo del Toro. The collaborating channels included Jesse Cox, Press Heart to Continue, Total Biscuit, Husky Starcraft, Game Grumps, Crabcat Industries, and The Game Station.
A prominent street sign in Hong Kong identifies the intersection of "Tull Street" and "Fong Street". Thomas Tull is the president of Legendary Pictures (which financed the film), and Henry Fong is one of the film's concept artists.
During the Hong Kong rampage, Gipsy Danger walks past a building with a lighted sign that says PLATINUM DUNE. Platinum Dunes is the name of a production company founded by Michael Bay, Andrew Form and Bradley Fuller.
Guillermo del Toro was inspired by the anime and tokusatsu (special-effects TV series and films) of his youth. He specifically cites Tetsujin nijûhachi-go (1963) as a major influence. Despite this, he wanted to avoid referencing other works of fiction in the design of the robots and monsters.