During the making of this movie, Jean Seberg was being spied on by the FBI. After her suicide in 1979, it was revealed that FBI Director J. Edgar Hoover had said that Seberg "should be neutralized." The FBI went so far as to deliberately and publicly humiliate Seberg by planting a rumor that the pregnant actress's unborn baby was fathered by a Black Panther.
In a letter to her parents in November 1969, Jean Seberg writes, "I've been offered a good part for good money in a western, to be shot entirely in Mexico, playing opposite David Janssen who, if I'm not mistaken, is one of Mom's favorites." (from the book "Jean Seberg - Breathless" published in 2008)
Since she spoke Spanish fluently, Jean Seberg acted as interpreter between the Mexican crew and the American cast.
Director Bernard Kowalski had worked with David Janssen on Janssen's first TV series, Richard Diamond, Private Detective (1956), some 13 years before this movie.
Diane confessed that during the shooting, when the crew ate during a noon break, Jean Seberg was so moved to see native people and a skinny dog starving that she permitted the dog to eat one piece of meat she took from her own plate. One member of the crew then told her not to do that, because every native would come and interfere with the crew job to get eating too. In response of this talking to her, Jean Seberg decided to starve for several days.