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.
Since she spoke Spanish fluently, Jean Seberg acted as interpreter between the Mexican crew and the American cast.
As with many of his co-stars throughout his career, Jean Seberg enjoyed working with David Janssen. She gave an interview in French where she said that he "had proved very amusing company." (from the book The Films of Jean Seberg, published in 2012)
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)
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.