"Runaway" was the first purely electronic score for composer Jerry Goldsmith. In order to give the synthesizers ambiance, several speakers were set up in a theater, and the playback recorded.
Many of the promotional images for the film (most notably the posters) featured Tom Selleck holding the rocket gun used by the villain, Luther, in the film, even though Ramsey never uses it during the story.
Gene Simmons, in his book "Sex Money Kiss", said that director Michael Crichton's casting of him was based primarily on this: Crichton asked Simmons to stare at him for about a minute without making any facial expressions. Apparently, Crichton decided on this that Simmons looked menacing enough and cast him for the role of Charlie Luther.
Tom Selleck's character's name "Ramsay" is said approximately 52 times in the movie; a little more than once every two minutes.