Rory Calhoun, who was in Italy for the title role in MGM's Marco Polo (1962), stepped into the lead role of "Colossus" on only one day's notice. He initially arrived at 11:30 on the first day he was to start filming. According to Leone biographer Christopher Frayling, Calhoun's first day included an accidental fall into a swimming pool.
During most of his scenes, Rory Calhoun wears a wide bracelet on his left arm and stiffly extends it covered by a cloak. According to Sergio Leone scholar Christopher Frayling and Calhoun's widow, the actor did that to hide some 20th-century tattoos.
According to Sir Christopher Frayling, Sergio Leone's early concept for the Colossus was to have its arms crossed and the visage of Benito Mussolini. This concept was abandoned and replaced with Helios/Apollo.