Actor Allusion: Jenner's death is similar to Tony Montana's, as both characters are hit in the back and then fall face down into a pool. Paul Shenar, who voices Jenner, played Alejandro Sosa in the same movie. However, this is an accidental example, as Scarface (1983) wasn't released until the year after The Secret Of NIMH.
Avoid the Dreaded G Rating: The reason the film was so dark was because Don Bluth wanted the film to appeal to a larger audience by getting a PG rating. Despite the dark themes, and one usage of the word "damn", the film got a G rating anyway.
Doing It for the Art: Several of the staff members had to work 110-hournote That's approximately 15-16 hours a day! weeks during production, and Bluth and a few other higher-ups had to mortgage their houses to pay for the film.
Fan Nickname: Elizabeth, as Mrs. Brisby's never-revealed first name, in tribute to her voice actress Elizabeth Hartman (who committed suicide). Some even go so far as to call her "Elizabeth Hartman Brisby".
No Budget: The film was made on 7 million dollars, which is far smaller than the budgets Disney movies were accustomed to having, and some of that budget was achieved by having Bluth and some higher-ups mortgage their houses. But because of the staff's resourcefulness and dedication, the film's presentation and art don't give the slightest trace of it being a shoestring effort.
The Other Darrin: Almost everybody from the first film that reappears in the sequel has new voice actors. Jeremy and Mr. Ages are the only characters from the first film that retain their original voice actors.
According to Don Bluth's biography, NIMH originally had a screenwriter write a draft for it that stuck closer to the book. Another early draft by Steven Barnes had considerable differences from the final film; the scientists of NIMH also had a bigger role in the earlier drafts, with the story cutting back between them and the animals—the idea of making one of the scientists be a main villain, counterbalanced by other "good" scientists was discussed (the film reduced their role to a flashback and off-screen presence), Mrs. Brisby was supposed to be even more emotional. There was also a scrapped new character, a female rat named Isabella. There was also going to be a scene where Dragon the cat attacked the Brisby home, and after the rats drove him off, Mr. Ages and Justin would explain to Brisby's kids and what happened to their father. A climax would have involved NIMH gassing the den, with Brisby and the other rats trying to escape it. There was also no amulet; the only true challenge in the climax was Brisby making a "Leap of Faith" jump over a deadly chasm with her kids, only making it because of encouraging words previously spoken to her by Justin.
The film's second draft had a scene involving Mrs. Brisby saving a beached sea bass, which impressed the rats. The climax originally had Brisby's children enter the abandoned lair of the rats—Mrs. Brisby runs off and saves them, only to be seemingly crushed by a cave-in. Jeremy, the children, and Mrs. Shrew would mourn her loss, but it turns out Mrs. Brisby is alive—the sea bass she rescued earlier helped her find a secret exit from the lair inside a pond. Surprisingly, the ending also implies that the Rats of NIMH never actually existed (both the children and humans can't find any evidence of their presence), and were a self-improving hallucination Mrs. Brisby was having.
To aid the subtext of the Great Owl and Nicodemus being one and the same, the idea of them sharing the same voice actor was considered, but they decided the film and cast needed more big name voice talent.
MGM actually asked Don Bluth Studios to work on the sequel, but due to heavy development on Anastasia, it had to be turned down (Bluth had actually spoke of early story ideas in which Timmy and Martin would play hero and villain, albeit in reversed roles from the final product).
One wonders what would have happened if, rather than the travesty they actually made, the creators of the sequel movie had actually adapted Racso and the Rats of NIMH. While the book had its problems (Retcon, including elements from the movie which contradicted the original book, some Gary Stu-ism regarding Timothy), in most respects it was a very satisfying and exciting book which showed how the rats of NIMH really would (and should) have designed Thorn Valley. It even handled the "Timothy is the hero who will save Thorn Valley" plot far better than the terrible sequel did—complete with a great deal of Not Now, Kiddo and How Do I Shot Web? before the rats actually listen to him and Racso and he come up with ideas that actually work.
Write Who You Know: Jeremy being re-written as a hopeless romantic was loosely based on a woman who cooked for Bluth's staff on the "Don't Walk Away" segment on Xanadu and repeatedly tried to flirt with Bluth.