Despite being listed on posters and having main title billing, Ursula Andress only has one line in the entire film.
Harry Hamlin and Ursula Andress began a relationship during production. Their son, Dimitri Hamlin, was born in 1980 after completion of principal photography.
Laurence Olivier was so ill during the making of the film, he would often go and lean on his tall, burly co-star Pat Roach, saying "Let me draw some of your strength, dear boy."
According to Harry Hamlin, prior to choosing to do this movie, he was considering doing another project with Richard Burton, a movie adaptation of the story of Tristan and Isolde that became Pecado Mortal (Tristán e Isolda) (1981). He ultimately gravitated towards what became this movie, despite not knowing anything about it, for the opportunity to work with Laurence Olivier, the one actor he considered even greater in stature than Burton.
Calibos, Lord of the Marsh and son of Thetis, does not appear in Greek mythology. He is based on Caliban, an antagonist William Shakespeare created in 1611 for "The Tempest." In Greek mythology, the son of Thetis was Achilles, Greece's best warrior in the fight against Troy.