Director Andrew V McLaglen felt Cahill US Marshal was "not the usual John Wayne movie. It's a very deep, personal story about children neglected by a father who is just trying to do his job." The star was far from being in good shape, having had a cancerous lung removed back in 1964. He was also struggling with emphysema on his remaining one.

Being significantly weakened, Wayne was forced to use a stepladder to climb onto his horse in the movie. As for riding shots from a distance, Duke's character was doubled by Chuck Roberson. When the film came out, people thought Duke was too young to play the dad of his on-screen sons. In real life, he had kids the same age as the actors Gary Grimes and Clay O'Brien.

Back in the days of the Old West, lots of men didn't have children until they were older. But Wayne was feeling elderly himself because his friend, director John Ford, was very sick.

When John Ford died in August 1973, Wayne said: "I'm pretty much living on borrowed time."

Hardly helping matters, Cahill US Marshal was the worst-reviewed of his films since he played Genghis Khan in 1956's The Conqueror. Produced by Duke's son Michael under his father's production company Batjac, Cahill had failed to live up to the praise Wayne received for his Oscar-winning role in 1969's True Grit.

Wayne later said about Cahill US Marshal: "It just wasn't a well-done picture. It needed better writing, it needed a little better care in making."

The Hollywood legend would go on to make a couple of better-received Westerns in True Grit sequel Rooster Cogburn opposite Katherine Hepburn and The Shootist. The latter film saw him playing a terminally ill gunfighter, while the star himself died of cancer just a couple of years later in 1979.