Sigourney Weaver's character was based on an object of infatuation that Steve Tesich had with a Washington, D.C. anchorwoman. He recorded her broadcasts and had pictures of her like William Hurt's character had in the film of Weaver. The actual anchorwoman was brought in by Peter Yates for technical support to make Weaver's character more believable.
In order to research his character, William Hurt actually moonlighted as a janitor, before production on the film began, and it would prove vital throughout the film.
After the film's poor box-office showing under the title of "The Janitor" in the UK, its title was changed to "Eyewitness" for further theatrical releases in new territories.
"The Janitor" was the movie's original title. The film's screenwriter Steve Tesich was once an office-building janitor himself.
The basement where Deever (William Hurt) works, and which plays a vital part in the film, was shot on a set built by production designer Philip Rosenberg, at Kaufman Astoria Studios in New York City.