Since all footage was filmed silent, Clint Eastwood did not add his voice to the soundtrack until 1967, when the movie was prepared for U.S. release.
Because this was an Italian/West German/Spanish co-production, there was a significant language barrier on the set. Clint Eastwood communicated with Sergio Leone and the Italian crew mostly through stuntman Benito Stefanelli, who also acted as an unofficial interpreter for the production.
Clint Eastwood's contract for Rawhide (1959) prohibited him from making movies in the United States while on break from the series. However, the contract did allow him to accept movie assignments in Europe.
Besides Clint Eastwood of course, actors Mario Brega, Benito Stefanelli, Aldo Sambrell, and Antonio Molino Rojo are the only actors to appear in all 3 of the "Dollars Trilogy" movies.
Filmed in 1964, but not released in the US until 1967.
This was the first time that Sergio Leone and composer Ennio Morricone worked together. Initially Leone was not keen on using Morricone for this film. Lacerenza's initial trumpet performance of the score made Leone quickly set aside any reservations. Leone and Morricone, who had known each other since 3rd grade, would develop a close working relationship that would last through all of Leone's future films.