Production notes

Immediately following the success of 1959's Some Like It Hot, Wilder and Diamond wished to make another film with Jack Lemmon. Wilder had originally planned to cast Paul Douglas as Sheldrake; however, after he died unexpectedly, Fred MacMurray took his place. The initial concept was inspired by Brief Encounter by Noël Coward, in which Laura Jesson (Celia Johnson) meets Alec Harvey (Trevor Howard) for a tryst in his friend's apartment, which ends up thwarted. However, Wilder was unable to make the comedy about adultery he envisioned in the 1940s due to Hays Code restrictions. Wilder and Diamond also based the film partially on a Hollywood scandal in which agent Jennings Lang was shot by producer Walter Wanger for having an affair with Wanger's wife, actress Joan Bennett; during the affair, Lang had used a low-level employee's apartment for trysts. Another element of the plot was based on the experience of one of Diamond's friends, who returned home after breaking up with his girlfriend to find that she had committed suicide in his bed. Although Wilder generally required his actors to adhere exactly to the script, he allowed Lemmon to improvise in two scenes. In one, he squirts a bottle of nasal spray across the room, and in the other he sings while cooking spaghetti (which he strains through the strings of a tennis racket). In another scene, where Lemmon was supposed to mime being punched, he failed to move correctly and was accidentally knocked down. Wilder chose to use the shot in the film. Lemmon also caught a cold (he was supposed to come down with in the script) when one scene on a park bench was filmed on a bitter autumn night.

Art director Alexandre Trauner used forced perspective to create the set of a large insurance company office. The set appeared to be a very long room full of desks and workers; however, successively smaller people and desks were used, ending up with children. He designed the set of Baxter's apartment to appear smaller and shabbier than the spacious apartments that usually appeared in films of the day. He used items from thrift stores and even some of Wilder's own furniture for the set.

The film's title theme, written by Charles Williams and originally titled "Jealous Lover", was first heard in the 1949 film The Romantic Age. A recording by Ferrante & Teicher, released as "Theme from The Apartment", reached #10 on the Billboard Hot 100 chart later in 1960.

The film made double its $3 million budget at the US and Canadian box office in 1960. Critics were split on The Apartment. Time and Newsweek praised it, as did The New York Times film critic Bosley Crowther, who called the film "gleeful, tender, and even sentimental" and Wilder's direction "ingenious". Esquire critic Dwight Macdonald gave the film a poor review, calling it "a paradigm of corny avantgardism". Others took issue with the film's controversial depictions of infidelity and adultery, with critic Hollis Alpert of the Saturday Review dismissing it as "a dirty fairy tale". MacMurray, having generally played guileless characters, related that after the film's release he was accosted by women in the street who berated him for making a "dirty filthy movie", and one of them hit him with her purse. In 2001, Chicago Sun-Times film critic Roger Ebert gave the film four stars out of four, and added it to his Great Movies list. The film critic Clarisse Loughrey has identified it as one of her two favorite movies, along with the 2010 film Boy. The film holds a 93% rating on Rotten Tomatoes, based on 103 reviews with an average rating of 8.8/10; the site's consensus states that "Director Billy Wilder's customary cynicism is leavened here by tender humor, romance, and genuine pathos". On Metacritic, the film has a score of 94 out of 100 based on 21 reviews, and was awarded the "Must-See" badge.