"Monco" is officially not the same character as "Joe" in A Fistful of Dollars (1964). This was the finding of an Italian court that adjudicated the lawsuit brought by Jolly Films, producer of A Fistful of Dollars. After the release of that movie, writer and director Sergio Leone had a falling out with the producers and made this movie with a different producer, Alberto Grimaldi. Jolly Films sued, claiming ownership of the "Joe" character, but lost when the court decided that the western gunfighter's persona, characterized by the costume and mannerisms, belonged to the public domain's folklore.
Lee Van Cleef was very grateful for his part in this movie, as he had fallen on hard times due to his heavy drinking. This movie effectively marked a resurgence in his career. Van Cleef had taken up painting in the interim as a way of making money.
The town of El Paso, designed by Carlo Simi in Almería, Spain, was the biggest set for which writer and director Sergio Leone was responsible at the time. It was reused for several scenes in The Good, the Bad and the Ugly (1966), in which it stood as several different towns. It's still standing to this day and is called "Mini Hollywood."
Gian Maria Volontè performed his lines in English, although he needed a translator to tell him everything word-for-word, as he did not speak the language. Contrary to some sources, his voice is not heard in the English version, in which his lines are dubbed by veteran voice actor Bernie Grant, who also voiced Volontè in A Fistful of Dollars (1964) and the Embassy Pictures version of A Bullet for the General (1967).
Clint Eastwood's paycheck had increased from $15,000 for A Fistful of Dollars (1964) to $50,000 for this movie, and reached $250,000 (plus 10% of the gross) for The Good, the Bad and the Ugly (1966).
Sergio Leone: [theme] Monco (Clint Eastwood), Colonel Douglas Mortimer (Lee Van Cleef), and El Indio (Gian Maria Volontè).