The film had a profound impact on both American and Soviet leaders. It is one of the few films actually mentioned in the Congressional record. Norman Jewison was also personally invited to Moscow, where he reported that the Russian crowd was transfixed by the scene featuring the little boy who falls from the bell tower, and the Soviets and Americans cooperate to save him.
Ordinary townspeople were used as extras in the film. They were so thrilled to be a part of production that the rushes were shown at the end of each day in a local theater. The townspeople went every night, bringing the entire family just to watch the rushes.
Unable to borrow a real submarine from the US Navy, and unable to bring a Soviet submarine to the United States, the art department built the Russian sub. It is powered by four motors underneath the hull, each driving a section of the "sub", and if you watch carefully, you can see it flexing where the four sections are joined.
Although the action in the film is supposed to take place on fictional "Gloucester Island" off the coast of New England, most of the outdoor scenes were filmed in Mendocino California. Mendocino in the 1960s was a somewhat remote artist colony on a rocky cape projecting into the Pacific Ocean, about 100 miles north of San Francisco. The harbor scenes were filmed in NOYO Harbor, just south of Fort Bragg, where Carine's Fish Grotto and Cappy's Bar still exist to this day. (2006)
Since the story takes place on the East Coast but was being shot on the West Coast, Norman Jewison decided to shoot the sunrises over the water at a precise moment just before dusk, after the sun was set over the Pacific, and did his best to augment pink colors in the final print.
Johnny Whitaker (spelled 'Johnnie' in the credits) so impressed Brian Keith that when TV's Family Affair (1966) began casting later that same year, Keith requested Whitaker test for the part of his nephew.