(at around 52 mins) As Cdr. Walker's mental condition worsens, he mutters "Fate has ordained the men who went to the Moon to explore in peace remain on the Moon to rest in peace." This line paraphrases the beginning of a contingency speech, drafted by speech writer William Safire in a memo entitled "In Event of Moon Disaster," which was intended to be delivered by President Richard Nixon in the event that the Apollo 11 astronauts were stranded on the lunar surface without hope of rescue.
This film was shot using old camera lenses from the 70s.
The prologue text at the beginning of the movie states that the documented footage of the secret Apollo 18 mission was uploaded to the website "www.lunartruth.com". When the movie was in theaters, if you typed that address into your browser, it would redirect you to "apollo18movie.net", which was the movie's official website. Both sites have since been shut down.
In some way, there was a real Apollo 18 during the year 1975. Named ASTP (Apollo-Soyuz Test Project), the mission was the first docking of spacecrafts built by different nations. The American crew included one of the seven Mercury program astronauts, Deke Slayton (the one who had never flown) and the Russian one Alexey Leonov, who was the first man to "walk in space."
The script was constantly rewritten during the shooting of the picture.