Christopher Plummer had played Pizarro in the 1965 Broadway run of the play; he was asked by Robert Shaw to sign on to the film as Atahualpa. Plummer drew inspiration for his own performance from David Carradine's stage depiction of the Inca.

Robert Shaw did extensive literary and experiential research before filming began, reading books about Pizarro's career, retracing the conquistador's steps through Perú and Spain, and visiting Pizarro's tomb.

Producer Philip Yordan also read extensively in support of the movie, and in Spain he received access to 16th century eyewitness accounts. A speech given during Atahualpa's trial was derived from the actual trial records.

Citing frustrations with typecasting and the infrequency of high-quality roles, Christopher Plummer wanted to concentrate on directing and had suggested that this movie might be his last film appearance.

Christopher Plummer swung a spear about for fifteen minutes a day in physical preparation for his role as Atahualpa.