Donald Sutherland insisted on performing his own stunts in the film's climax. His scenes at the pod factory were filmed without harnesses or nets. In the shot of a fireball erupting from the factory, Sutherland barely missed it. However, an extra missed his cue and was seriously injured from the explosion.
The leather half-glove that Leonard Nimoy's character wears was deliberately used for the sole purpose of making the character more distinctive and recognizable. Nimoy got the idea from a friend who wore it to cover a burn on his hand.
Among the sounds Ben Burtt used for the pod growing scene, the heartbeat came from an ultrasound recorded on his pregnant wife. The pod screams were recorded pig squeals. Additionally, the natural diegetic sounds (crickets, birds chirping) fade as the film progresses, until only mechanical sounds (sirens, the garbage trucks) are heard.
Matthew's story/joke goes as follows. The British are trapped in the Sahara and are surrounded by the Germans. One day, an officer makes an announcement: "I have good news and bad news. The bad news is, we have no food but camel poop. The good news is, there is plenty of it."
At the beginning of the film, as the alien spores rain down on earth, you see them presumably landing on the Transamerica Pyramid in San Francisco - the headquarters of what was then the parent company of United Artists, which produced the film.
Robert Duvall, who had previously worked with Director Philip Kaufman on _The Great Northfield Minnesota Raid (1972)_, happened to be in San Francisco at the time of filming and shot his only scene for free.
During the mud bath scene a man suggests that Nancy read a book titled "Worlds In Collision" by Immanuel Velikovsky. The book was published in 1950 and spent eleven weeks on the New York Times bestsellers list. However, the book was met with overwhelming rejection of its thesis by the scientific community.
The trivia item below may give away important plot points.
Only Philip Kaufman, W.D. Richter, and Donald Sutherland knew how the film was going to end. Veronica Cartwright was not told that Sutherland's character had been captured and became an alien. When they filmed the ending in front of San Francisco City Hall and Sutherland pointed to her, imitating the pod scream, Cartwright's reaction of cold fear is authentic.