This was the only cinema film to be directed by S. Lee Pogostin, a well-known television writer. James Coburn later claimed in interviews that Pogostin was the cause of the film's considerable box-office failure, as he had refused to alter his extremely wordy script and then proved to have little idea of how to direct a film. According to Coburn, the actors more or less directed themselves whilst cameraman Jack Hildyard handled the technical details.
When Cunningham is in Brussels a fight breaks out in the bar over the issue of whether the official language of Belguim should be French or Flemish (a Dutch dialect). Belgium is a country roughly divided half-and-half between the two languages.
When Cunningham thinks that Sheila is a prostitute he pays her 6,000 pesetas. That amount would have been equivalent to $85.72 in 1969; after adjusting for inflation, that would be equivalent to $560 in 2015.