This story was conceived when Garson Kanin, trying to cheer up his wife Ruth Gordon, was driving by Columbus Circle. He told her he was going to put her name on "that billboard there" in the biggest letters. He didn't. He wrote a screenplay instead. Gordon suggested that the lead should be Judy Holliday. Kanin had originally considered a male lead, Danny Kaye. When he finished the screenplay, the lead had been written for Holliday.
In a 1972 interview, George Cukor told Gavin Lambert about the little natural moments that come out in performances - as an example he described the shooting of the seduction scene in Adams' apartment. "It so happened we had a property man on the picture who'd worked with The Three Stooges. He said, "I have an idea, may I help on this?" I said, "Please do," and he suggested, "Let her take the earring off herself, so he can nuzzle her ear." So we did, and it made a terribly funny moment. Later in the scene she had to pour champagne down Peter Lawford's neck. We only have four shirts for Peter Lawford, so we could only shoot four takes, and it was tricky for the camera. On the last take I said, "Judy if you laugh, I'll just kill you, I'll kill you dead." Well, she didn't laugh, but she giggled, and it was absolutely great. I asked if she'd done it deliberately, in spite of what I'd said, and she didn't really know. Sometimes you get these very human things on the set."
Garson Kanin originally wanted to direct his own script but could not get a commitment from studio chief Harry Cohn in writing guaranteeing him final cut. He ultimately sold the story outright to Cohn and went to Europe for three years.
Jack Lemmon, who had previously only acted briefly in television, had a tendency to overact for the camera but George Cukor soon convinced him that "less is more." The actor later remarked, "I've learned my craft from that advice. It's the hardest thing in the world to be simple, and the easiest thing in the world to act your brains out and make an ass of yourself."
George Cukor saw the importance of the location filming, especially in regards to the real-life background characters: "We used Central Park as a character, as we did in The Marrying Kind (1952), and this time it was during a heat wave, which brings all the mad people out. You can see lots of mad people in the park and sitting on steps in front of houses."
A perfect example of George Cukor's approach to acting was demonstrated to Jack Lemmon during a restaurant scene where Pete and Gladys argue. Cukor recalled, "They rehearsed it and did it very well, but I said, "I don't believe it, I don't believe one damn thing. Jack, what do you do when you get angry?" He said, "I get chills and cramps, I get sick to my stomach, but can't use that." "Oh," I said, "do that!" So in the height of fury he suddenly clutches his stomach, and it makes all the difference."
The film features extensive location shooting in New York City, providing the level of realism that the story demands (especially given Pete's profession as a documentary filmmaker). Many dialogue scenes take place outdoors, and studio shooting against rear projection screens is apparent only in close-ups.