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 their entire families with them.
Brian Keith spoke fluent Russian which he displayed in Meteor (1979) and World War III (1982). Here, Keith plays the Police Chief of Gloucester Island and has Alan Arkin translate his formal complaint from English to Russian to the Submarine Commander. Arkin also spoke Russian fluently, which he had learned from his Russian grandparents.
Theodore Bikel speaks only Russian in the film. Norman Jewison has acknowledged that his fluency in the language was a principal reason in casting him.
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". It can be seen 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 Cappy's Bar still exists in 2023. Carine's Fish Grotto closed in 2022.