Laurence Olivier was 71 years of age when he made this film. He was recovering from thrombosis and pneumonia. Reportedly, Olivier once said that his teenage co-star in this film, Diane Lane, was "the next Grace Kelly".
The place where the young couple wanted to seal their union with a sunset kiss was under Venice's Bridge of Sighs. This was based on a legend which said that "lovers who kiss beneath the Bridge of Sighs in Venice, at sunset as the bells of Campanile toll, will love each other forever".
Laurence Olivier wanted to do his own stunts despite his age and health. Director George Roy Hill had a specially designed bicycle manufactured for him. It had a motor that drove it, making the cyclist look like he was pedaling it. However, Olivier wouldn't stand for this, and when Hill was away he got on the real bike and rode down a hill all on his own, proving his cycling competency.
Filming in Paris, where he had been part of the U.S. Army when they liberated the city in 1944, was a great experience for Broderick Crawford. According to co-star David Dukes, "He could still walk into restaurants where the owner would remember him and sit him down to dinner."
Thelonious Bernard spoke very little English and had no prior acting experience. Prior to principal photography, he spend a month at the home of the film's director, George Roy Hill, where he did a crash course in acting and English.
Claude Klotz wrote a sequel to the film's source novel called "Pythagore, je t'adore" ("Pythagoras, I Love You"). The book was published in 2001, but to date [January 2013] has never been filmed. The plot starts several years after the events of the original story. In the story, Lauren King has moved back to the US and is not in contact with Daniel Michon anymore, but they reconnect.
The scenario is based on a novel: "E=mc2 Mon Amour" by Claude Klotz (writing under the better-known name of Patrick Cauvin) which had been first published in 1977, only two years after the death of Patrice des Moutils (aka "Monsieur X", a probability expert working in an insurance company) who became famous in France during the early 1960s for his creative use of computers. Des Moutils had the idea of calculating the Bayesian probabilities of winning for each and every horse in a race, and placing accordingly an array of bets. This method brought him fabulous gains (20 times the amount of his bets) but he became subject to the unremitting hostility of the French OTB system which succeeded--after several years of harassment--in bankrupting him and thus inadvertently driving him to suicide.
Julius and Lauren quote Elizabeth Barrett Browning's "Sonnet #43" from the Sonnets from the Portuguese. It begins with "How do I love thee? Let me count the ways." The two recited the last lines, which are "and, if God choose,/ I shall but love thee better after death."