When Dr. Herbert Bock rants, "We have established the most enormous, medical...entity ever conceived and people are sicker than ever!" the slight pause, searching for the word "entity", was spontaneously ad-libbed by George C. Scott to save the take. The scripted line was, "we have ASSEMBLED the most enormous medical ESTABLISHMENT ever conceived." Scott heard his slip in mid-sentence, so he reworded the line so as to not make it repetitive. Director Arthur Hiller loved the save so much he used that take in the movie.
George C. Scott was the first choice for Dr. Herbert Bock, but he almost lost the part when he demanded a $300,000 salary, which was a high pay at the time. Rod Steiger was then offered the part, but when he asked for even more money, the producers cast Scott after all.
Even though George C. Scott had declined an Oscar® nomination for The Hustler (1961) and had rejected the Best Actor Academy Award® gold statuette he had won the previous year for Patton (1970), Scott was still nominated in the same category for this movie, but lost to Gene Hackman for The French Connection (1971).
According to Turner Classic Movies host Robert Osborne, screenwriter and producer Paddy Chayefsky passed on having Walter Matthau or Burt Lancaster in the role of Dr. Herbert Bock, and either Jane Fonda, Ali MacGraw, or Candice Bergen in the role of Barbara Drummond.