William Wyler first considered making this film as a follow up to Roman Holiday; as in that film, Gregory Peck would have played the male lead opposite Audrey Hepburn. At that time he had a darker mood in mind and approached Stanley Kubrick, who had recently made The Killing, to contribute.
George C. Scott was originally cast as Leland, but was fired by William Wyler when he showed up late on his first day of work. He was replaced by Eli Wallach.
After Nicole (played by Audrey Hepburn) dresses up as a cleaning lady, Simon Dermott says "That does it. For one thing, it gives Givenchy a night off." Hubert de Givenchy was Audrey Hepburn's costume designer.
Nicole's car is a Autobianchi Bianchina Special Cabriolet (basically an upmarket Fiat 'Sport' 500). Dermott's car is an E-type Jaguar.
The book Nicole is reading in bed ('Hitchcock Magazine: Le Revue du Suspence') is a French version of the 'Alfred Hitchcock Mystery Magazine', first published in 1956.
The police escort vehicles featured in the beginning sequence are as follows: a 1965/66 Citroen DS-21 unmarked police sedan, a 1965 Citroen Type H Police van, and a '65 Citroen H delivery van. The Citroen DS-21 sedan, designed by famed industrial designer Flaminio Bertoni, is a particularly noteworthy addition to this movie as its futuristic and aerodynamic signature concept design, along with its innovative technology - the first mass-produced car to feature front disc brakes - caused the DS to be named "the most beautiful car of all times" by Classic and Sports Car Magazine.
The portrait that Dermott points out to Nicole as a "superb Rembrandt," is "A Portrait of Jacob Trip", painted in about 1661.