Shirley MacLaine and Robert Mitchum began a love affair that lasted for years during the shooting of this film. Mitchum and MacLaine continued their affair all over the world, traveling together to locales such as New Orleans, New York, London, Paris, and even West Africa. The relationship, however, would end after a couple of years, with Mitchum returning to his wife, and MacLaine to her husband, Steve Parker. In her memoirs, however, MacLaine recalled a conversation years later with Used People (1992) costar Marcello Mastroianni: "We laughed about the time he and Faye Dunaway, who believed they were being successfully discreet, ran into Robert Mitchum and me on a London street. We believed we were being successfully discreet. And so the conversation led to the dilemma of falling in love with one's costar. "One must love one's costar," said Marcello. "Otherwise how will the audience believe it?"
Based on the Broadway play written by William Gibson, which starred Anne Bancroft and Henry Fonda. They were the only actors in a two-character play; the cast was greatly expanded for this film version.
Giving a drunken promotional interview for Ryan's Daughter (1970) to the young Roger Ebert in 1969, Robert Mitchum vented his low opinion of director Robert Wise: "Bobby even times a kiss with a stopwatch. He marks out the floor at seven o'clock in the morning, before anybody gets there. Lays it all out with a tape measure. True. It's very difficult to work that way. I worked with him and Shirley MacLaine, and Shirley said, 'Why doesn't he go home? He's just in the way...' "
Elizabeth Taylor and Paul Newman both were set to do this film, but when Taylor became ill during the early filming of Cleopatra (1963), Newman was able to do The Hustler (1961) instead.
The original Broadway production of "Two for the Seesaw" by William Gibson opened at the Booth Theater in New York on January 16, 1958, ran for 750 performances, and was nominated for the 1958 Tony Award for the Best Play.