Sigourney Weaver's character was based on an infatuation that writer Steve Tesich had with a Washington D.C. anchorwoman, in which he recorded her broadcasts and had pictures of her like William Hurt's character had in the film for Weaver. The anchorwoman in question was brought in by Peter Yates for technical support on the set to make Weaver's character more believable.
According to director Peter Yates, the script for the film was a hybrid of two screenplays that Steve Tesich had written and had gotten nowhere with either one. It was Yates who suggested that he combine the two scripts, Tesich originally balked at the idea, but finally gave in and this script was formed. Yates also said Tesich had trouble coming up with unusual plot twists because he cares more about character than plot when he writes.
The film's original title during shooting was "The Janitor Can't Dance", which is the title that both Peter Yates and Steve Tesich liked, but Twentieth Century Fox forced them to change the title during post production, at first to The Janitor (the U.K. title for the film) and finally to Eyewitness, a title Yates to this day still doesn't like.