Jacqueline Bisset is partially dubbed by another actress. Soon after filming, Bisset received a role in a major Hollywood film. She left England and was therefore unavailable when it came time to re-dub some of her lines.
Audrey Hepburn and Albert Finney were required to "direct" themselves in several sequences where they were shown inside of a moving automobile. They pushed the camera's buttons while director Stanley Donen rode alongside them in another car.
Several biographies of Hepburn state she was very nervous shooting her first nude scenes for this film, particularly a skinny dipping sequence. If such a scene was shot, it didn't end up in the final print.
Audrey Hepburn was Stanley Donen's first and only choice for the role of Joanna. When he sent her a treatment, she turned it down because the avant-guard style of filmmaking didn't appeal to her, as she had already done one like it before that hadn't been successful (it was probably Paris When It Sizzles (1964)). When he tried again and sent her a whole script, she loved it and agreed to do it.
Audrey Hepburn was afraid of water, so the scene in which Mark throws Joanna into the water had to be done with some divers standing just barely outside the camera range. One take was ruined because the diver jumped in too save Audrey too quickly.
The poem recited to Caroline in the car, "The bumble-bee, the bumble-bee/ He flew away from the tulip tree..." is from 'Tommy's First Speaker for Little Boys and Girls' by Thomas W. Handford, published in 1886
Average Shot Length = ~4.5 seconds. Median Shot Length = ~4.4 seconds. Both values are quite fast, and demonstrate the experimentation with faster cutting rates during the mid to late 1960s. For example, even a noted fast cutter like Sam Peckinpah would take until 1969 to produce a film (_Wild Bunch,The (1969)_) that is cut faster than _Two For The Road (1967)_.
The trivia item below may give away important plot points.
According to Stanley Donen on the DVD audio commentary, nobody wanted to film the scene where Mark confronts Joanna's infidelity at the cliff side café because many found it personally painful and realistic.