Most films have their composers put music to images far into post-production. These days, temp tracks are common right until the last possible moment. But such was Leone’s faith in his collaboration with Morricone, which had already spawned the hit ‘Dollars’ trilogy, he had him compose the key themes before filming – and played the music during the shoot in order to inspire his cast.

https://www.scotsman.com/read-this/once-upon-a-time-in-the-west-at-50-how-ennio-morricones-music-powered-a-classic/