Former J Pop singer Yoshiko Hidaka who also starred in Black Rain has a role in the film.
Katsuhiko Sasaki, who played Goro the inventor and Ichinose in the Showa series has a bit part as some random military personnel. He gets a bigger role in Godzilla vs. King Ghidorah though.
What Could Have Been: The film was originally written by a Japanese Dentist and featured a far different story than what the film is today. The film was to focus heavily on Dr. Shiragami as he creates both Biollante and a rat/fish hybrid monster named "Deutalios" out of insanity due to her daughter Erica's death. Godzilla would've fought, killed, and eaten Deutalios and would've fought Biollante until retreating due to injuries. However it was felt that Godzilla was given very little focus in the story, and so re-writes were made: replacing Deutalios with the Super-X2 and a rose form of Biollante, and Godzilla being injected with Anti-Nuclear Bacteria to slowly kill him.
In another ending, a losing Biollante would have tried to kill Godzilla by turning into a wave of pure energy.
One idea for Biollante's final design in the film was a flower with human face in the middle that talked.