A full-size model of the Guardian of Forever was built by makeup artists Tim Vittetoe and Shawn Shelton. However, it was digitally replaced by VFX artist Henry Gibbens because it looked "too small" on camera.
All the dialog from the Enterprise-M scene had to be "looped" - or re-recorded by the actors. It was raining all day during that scene and the Enterprise sets in upstate New York had a tin roof.
Apollo 11 astronauts Neil Armstrong and Buzz Aldrin were both offered roles as Admirals on the Enterprise-M vessel. Both were very interested, but neither space pioneer could fit the shoot into their busy schedules.
In the marriage scene between Uhura and Stonn (Lawrence Montaigne), the priestess is played by Arlene Martel, who appeared in "Amok Time" (1967) as T'Pring, Spock's fiancée. Spock rejected her after challenging him and she ended the episode married to Stonn.
The Enterprise-M Communications Officer is portrayed by Dr. Seth Shostak, who is a Senior Astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute.
The Guardian of Forever planet scenes were shot at Vasquez Rocks Natural Area Park north of Los Angeles. It was also the location for several Original Series episodes, including "Arena" and "Friday's Child."
The scenes on Vulcan were shot at a private residence in the Lakeview Terrace area of Los Angeles. The home was designed and built by a Disney "Imagineer" and was full of unique artwork and statues. The camera crew had to avoid shooting many of the statues, which were recognizable Disney characters. And, even though Vulcan is a desert planet, this lush location was used to represent an artificial Vulcan-made oasis.
When the final scene in the film was being shot, the crew realized no one had created the tribble that was supposed to appear in the scene. The crew finally resorted to using the furry-looking windsock off the boom microphone, which they pushed up in a box which had the bottom cut out. Actress Grace Lee Whitney moved her fingers inside the windsock to make the "tribble" appear to move.