Gary Goldman stated that they originally hoped the film would receive a "PG" rating, due to several intense scenes and the maturity of the subject matter. They were actually surprised when the ratings board gave it "G", and feel that the rating might have hurt the film's commercial appeal to older audiences.
The lead character's name was changed from Mrs. Frisby (in the novel) to Mrs. Brisby to avoid legal entanglements from the Wham-O company (makers of the Frisbee). Unfortunately this change came late in the film's production, long after the actors had recorded their dialog. Because it was not feasible to have every actor using the word "Frisby" in the movie re-record his or her lines, the change from "Frisby" to "Brisby" was actually made by the sound editors, who, by hand, carefully sliced the "br" (taken from other words spoken by the actors) into the "fr" on the magnetic dialog tracks.
Don Bluth, John Pomeroy and Gary Goldman all left Disney to pursue this project, which had originally been rejected by their former employer as "too dark" to be a commercial success. They were followed soon after by 20 other Disney animators, dubbed "The Disney Defectors" by the trade press.
This was Jerry Goldsmith's first music score for an animated film. He later said that it was among his personal favorites. He was instrumental in introducing the film to Steven Spielberg, who went on to work with Don Bluth on An American Tail (1986). According to Bluth and Gary Goldman in their DVD commentary, Goldsmith so loved the film that he volunteered an extra three weeks to polish and refine the score, even though he was not contractually obligated to do so.
The film was originally budgeted at $6.5 million, but was reduced by the studio after production was underway. Gary Goldman and the film's producers resorted to mortgaging their homes in order to raise the extra $700,000 needed to complete the film. In spite of these difficulties, the film still cost under $7 million - roughly half of what Disney had been spending at the time on each of their animated features, even with their cost-cutting methods in animation.
The staging of the climactic sword fight between Justin and Jenner was largely taken from the Errol Flynn / Basil Rathbone duel in The Adventures of Robin Hood (1938) (some choreography, including the villain getting stabbed in the stomach, was literally copied by the animators). Other moments were inspired by fight scenes in The Vikings (1958).
The movie also heralds a return to using the multiplane camera for scenes requiring depth, especially Nicodemus' magic hologram and in the opening sequence where with the aid of backlit animation, the wise old rat beckons magic vapors from an inkwell to grace the pages of an ancient book with fiery gold lettering.
Members of the crew did some of the crowd voices for the rats of NIMH. Even Norbert Auerbach, then Chairman of United Artists Europe, and Charles Champlin, a leading critic at The Los Angeles Times, recorded some incidental voices. Regrettably, when the film was released, Champlin said he couldn't review it due to a conflict of interest.
There are 1078 backgrounds in the film, all of which were tested and then shot in continuity to time so that the entire film track could be viewed with just the color backgrounds. This enabled the overall impact of the color scheme to be evaluated and some backgrounds were repainted as a result.
Techniques used on this movie which had not been used extensively in other animated features included split exposures to create shadows and translucency, diffusion in conjunction with split exposures to create reflections, color Xerography for the creation of cels and painstaking color orchestration. Newer techniques included the use of video animation for testing and backlight. The use of multiple exposure techniques was probably the best indication of the time and effort taken with the film.
In late August 1982, Disney barred several theaters from booking this film as a double-feature with their own summer offering TRON (1982). Disney claimed that the studio wanted "TRON" to be paired with another Disney film, but a Los Angeles Times article was convinced that the action was due to bitter feelings over Don Bluth's exit from the company.
Supervising Effects animator Dorse A. Lanpher was responsible for most of the effects scenes. He and his tiny crew of three effects animators managed to fill the film with the needed sparks, fire, shadows and water. The multiplane techniques he adapted for the project include a pseudo-hologram and the use of backlit animation making every dewdrop sparkle or supernatural amulet glow with a brilliance never seen before in animated films since Fantasia (1940). Shortcuts in the film include the photographing of three-dimensional model sets and objects for transfer to animation and the xeroxing of individual cels although, unlike the later Disney films, the cels are linked by hand to eliminate the animators' original sketchy lines. These time-saving devices do not, however, incorporate the use of TV's limited animation techniques or total rotoscoping.
Six months were devoted to developing the design specifications for the cameras Bluth and his crew needed for the newly founded Don Bluth Productions and their upcoming movie, and then another fourteen months were spent building and testing the cameras. Two identical camera stands were built by Mechanical Concepts especially for Bluth because there were no animation cameras available commercially that could do everything necessary to shoot the film, according to Fred Craig, Director of Special Processes for the film. One of the features which the cameras built for Bluth had that was not found on conventional animation stands was the capability of shooting backlit art in an anamorphic format.
Despite being rated G, Justin uses the word damn after Mr. Brisby gets caught by the farm boy inside the house. Later on in the word damn would be used in the third book sequel "R.T. Margret and the Rats of Nimh" by Jane Leslie Conly Published in 1990, The school bully would swear at Margaret when he questions her about Thorn Valley.
Originally Mr. Brisby was to encounter a female rat named Isabella whom she was to meet in a library, she was to explain the origins of the rats of NIHM, she also served as the nurse and was Justin's girlfriend, she was ultimately cut from the film.
The trivia item below may give away important plot points.
The film greatly deviates from the original novel in which there are no supernatural elements, and Nicodemus is neither magical or elderly, but merely an intellectual. Jenner is not a villainous character, but the leader of defectors from The Plan, and who is believed killed during an unsuccessful raid of a hardware store (that is how NIMH discovers the location of the rats). The move of Mrs. Bisby's home is accomplished by the rats without incident (or magic).