Robert Redford loathed wearing a suit and tie all day, which was required for his character. During breaks between filming, he wore western boots and a black cowboy hat.
This is one of Jane Fonda's favorites of her own films. She once tweeted, "too bad we never got Neil Simon to do a sequel... those characters 40 years later."
The film role of 'Corie Bratter' was originally offered to actress Natalie Wood, who already had played opposite Robert Redford in two movies. Wood declined the offer though, because she wanted to take time off.
A running gag is the absence of an elevator to get to the newlyweds' 5th-floor (plus stoop) apartment; new arrivals are out of breath throughout the film. For its release in France, however, the dubbed dialogue placed the apartment on the 9th floor (equivalent to the 10th floor in the US), since in France, older buildings with six stories and no elevator are not uncommon and audiences wouldn't have understood why climbing the stairs was so arduous.
Robert Redford had effectively given up acting prior to filming this movie. Disillusioned after the relative failure of Daisy Clover (1965), La poursuite impitoyable (1966), and Propriété interdite (1966), he took a break from Hollywood and spent a year traveling around Greece. It was only after Paramount threatened to sue him over certain "contractual obligations" that Redford returned home to star in this movie.