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"The Purgatory Cove" scene was shot in late September. To keep warm in the cold water, both Doug McKeon and Henry Fonda had to wear wet suits under their clothes. However, the water level was so low that they could have easily stood up and been only knee-deep in the lake. Katharine Hepburn was supposed to have a stunt double perform her "dive-in" scene for her, but instead she insisted on doing it herself. She dived into the frigid water without a wet suit. Apparently, Hepburn was familiar with swimming in cold waters on the eastern coast of the USA and regularly showered in cold water.
The brown Fedora worn by Henry Fonda belonged to Spencer Tracy and was given to Fonda by Katharine Hepburn on the first day on the set. Fonda, overwhelmed with the gesture, painted a still life watercolor of the three hats he wore in the film and gave the original to Hepburn as a gift. He had 200 lithographs made of the painting and sent one to every person who worked on the film. Each copy was numbered and personally signed by Fonda thanking each person by name. In her autobiography, Hepburn wrote that she gave the painting to screenwriter Ernest Thompson. After Fonda's death, she found the painting to be a sad reminder of him and Tracy.
Apparently, stars Henry Fonda and Katharine Hepburn had not only never worked together, but had never even met prior to working on this picture, despite having mutual friends and acquaintances.
Leftover footage of Henry Fonda and Katharine Hepburn driving around the New Hampshire countryside, as seen in the opening credits, was used for the opening of the 1982-90 CBS sitcom Newhart (1982).
James Stewart had wanted to star in the movie, but Jane Fonda bought the rights to the play before he could.
During a pivotal scene for the father/daughter, the camera was rolling for that close-up, "I reached out and placed my hand on his arm as I said, 'I want to be your friend.' What I saw amazed me: for a millisecond he was caught off-guard. He seemed angry, even: This isn't what we rehearsed. Then the emotions hit him, tears came to his eyes, then anger again as he tensed up and looked away....I loved him so much just then. It amazes me what a great actor he was in spite of his fear of spontaneity and real emotions." The take appears in the final film.