In the novel, Roy Grenville was British and was renamed Grant in the script. Robertson, a U.S. actor, was cast because he was popular internationally at the time and because an American central character improved the production's access to finance and worldwide audiences. Additionally, Robertson was an experienced pilot, owned a Supermarine Spitfire, and was personally interested in making the film as an accurate portrayal of wartime flying. Although he was refused permission to fly for the purposes of the film, his scenes stand out as a realistic depiction of operational flying.