Ascended Meme: The pairing of Charles Boyer and Irene Dunne created an unexpected Squee! from audiences, and the Los Angeles Examiner even called them (not their characters) the most romantic screen couple ever. Hollywood definitely noticed the reception because publicity stunts ensued.
Soon after the film entered theaters nationwide, the Hollywood fan magazine Photoplay featured two articles ("Charles, as Seen by Irene Dunne" and "Irene, as Seen by Charles Boyer") in which Dunne and Boyer basically wrote Loving Details about each other that guaranteed to make readers ship them.
Later that year, Dunne and Boyer starred as a screen pair in the romantic drama When Tomorrow Comes but it wasn't as successful, mostly because critics and audiences were still busy raving about Love Affair and frequently compared the two. When Tomorrow Comes was unlikely an attempt at Pandering to the Basenote Dunne and Boyer would have certainly made the film before Love Affair was released but the fact the third (and last) film they made was named Together Again (1944) mightn't have been a coincidence; its tagline even said "Their gayest and most exciting love affair!"
The Cast Showoff: Terry performing "Sing, My Heart". A frequent motif in Irene Dunne's movies is performing to an audience, having begun her acting career in musical theater (she had originally wanted to be an opera singer). In this case, it is exploited by Leo McCarey, a close friend, who made her sing in his movies.
Terry's accident was originally not part of the story, but enforcers of The Hays Code argued her adultery shouldn't be condoned. Becoming paralysed was approved.
The French embassy is said to have caused filming to halt for a few days. It was concerned the womanizing exploits of the movie's leading man would cause the United States to reconsider being France's ally if Nazi Germany decided to start a war.
Fake Nationality: Maria Ouspenskaya portrays Michel's French grandmother, despite being from Russia.
Playing Against Type: Irene Dunne, fresh off the success of a few screwball comedies, surprised critics with her tear-jerking performance as Terry. (However, Dunne had appeared in a variety of film genres before screwball comedies.)
Production Posse: This would be the second film that Leo McCarey made with Irene Dunne. For Dunne herself, it was the first of three films that she would act alongside Charles Boyer.
The movie was originally a period piece set in the 1850s, but French embassy interference made McCarey reconsider a modern plot.
Charles Boyer suggested Michel's grandmother should have a larger impact, so the script obliged.
Plans for a remake were reported in 1952. Fernando Lamas and Arlene Dahl were attached to the project and apparently composing new music.
Despite being first (and only) choice for Michel, Charles Boyer had been offered Good Girls Go to Paris by Columbia Pictures. One can only wonder whether Love Affair would've taken off if Boyer had taken Columbia's offer.
Terry McKay is the name of the leading lady throughout all three movies. The leading man's name changed from Michel Marnet to Nicolo "Nickie" Ferrante, to Mike Gambril.
There were two audio adaptations of the movie recorded by Lux Radio Theater (1940 and 1942). Irene Dunne reprised her role in both, but Charles Boyer reprised his role in the second version, whereas William Powell performed Boyer's role in the 1940 version.
Terry is apparently based on an American passenger 1939's screenwriter Delmer Daves met on a ship, who might've been the mistress of a small-town government official. She was returning from Europe after fleeing the country.