(at around 5 mins) Robert Duvall, who had previously worked with Philip Kaufman on La légende de Jesse James (1972), happened to be in San Francisco at the time of filming, and shot his only scene for free. He plays a crazy priest playing on a swing. It's supposed to foreshadow the pod person anomie and alienation we feel later in the movie.
Donald Sutherland was hit by a Volkswagen beetle while filming a shot of Matthew and Elizabeth running. He fell onto the windshield and was able to see the driver saying "Oh God, not you!"
(at around 1h 24 mins) During the taxi ride, Donald Sutherland and Brooke Adams's nervousness is genuine. Don Siegel had lost much of his vision and was driving through the dark streets of San Francisco without his glasses.
The leather half-glove that Dr. David Kibner (Leonard Nimoy) wore 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.
Michael Chapman: (at around 1h 26 mins) Janitor at the Health Department. Appears when Elizabeth cries in Matthew's arms and, later, when they sneak into the building.
Philip Kaufman: (at around 1h 5 mins) Playing an impatient man rapping on the window of a phone booth occupied by Donald Sutherland.