Louis Calhern replaced Frank Morgan in the role of Buffalo Bill Cody aka Buffalo Bill after Morgan died of a sudden heart attack shortly after filming had begun. But if you look closely at Buffalo Bill's very first appearance on his horse, you will see a second of Frank Morgan before the shot of Calhern.
Judy Garland and Frank Morgan, who appeared together in The Wizard of Oz (1939), were scheduled to re-appear in this film. Garland was fired because of health problems, and Frank Morgan died shortly after filming began. As a result of this, Betty Hutton took over Judy Garland's role as Annie Oakley, and Louis Calhern succeeded Frank Morgan as Buffalo Bill Cody aka Buffalo Bill.
Betty Hutton and Howard Keel did not get along during filming. Keel thought that Hutton cared more about her career than her co-stars.
Writers Dorothy Fields and Herbert Fields and Producers Richard Rodgers & Oscar Hammerstein II had originally arranged for Jerome Kern to write the musical score. After Kern died suddenly, he was replaced by Irving Berlin. This was the first time that Berlin wrote a score for a show with an existing plot. In his many musical plays and films, the songs were written first, then the scripts were written for the situations suggested by the songs.