Even though her character is presented as an elderly, retired woman, Shelley Winters was only 51 years old when the film was made. Ernest Borgnine was actually three years older than Winters, despite the fact that his character treats hers like a old woman.
Stella Stevens said in a 2006 interview that when she first read the script, she could tell by the writing that whichever actress played the "fat lady" (Belle) in the movie would end up getting an Oscar nomination. And she was right. The role went to Shelley Winters, and she received an Academy Award nomination for Best Supporting Actress.
Shot in sequence, taking advantage of the fact that the principals became dirtier and more tattered and suffered injuries - some real and some artificial - as they progressed.
Red Buttons and Carol Lynley, whose characters fall in love in the movie, actually disliked each other intensely during filming. They refused to have anything to do with each other, except when the cameras were rolling. Ironically, after being constantly reminded of this, they ended up becoming great friends in later years. Both Lynley and Pamela Sue Martin were with Buttons at the time of his final public appearance - the world premiere of Poseidon (2006) at Mann's Chinese Theatre in May 2006.
While he and the others are lifting the giant Christmas tree, Rogo mutters, "Holy fuck, it's heavy." This was a genuine reaction from Ernest Borgnine and the line was kept.