“When the music ends, pick up your gun.”

An American voice artist speaks for an Italian actor playing a Mexican bandit. Mid-20th-century Spain stands in for the American Southwest circa 1870. The music that the bandit refers to is a watch-chime melody that’s sonically rich and reverb-drenched beyond the mechanical abilities of any 19th-century timepiece. The music will end, but only after it has cresendoed into a fully orchestrated explosion that exposes the heart of “For a Few Dollars More” — Sergio Leone’s 1966 cinematic fresco of greed, regret, revenge, and death — like a symphonic retractor.

That music is, of course, the work of composer Ennio Morricone. In “For a Few Dollars More” an unlikely yet perfectly assembled amalgam of surging strings, a chanting male chorus, stark percussion, choral bells, staccato electric guitar, and a church organ that sounds as if it’s piped through Mount Vesuvius swells and ebbs, underscores and at times overshadows Leone’s vision of history, as theater and myth.

The American country-rock duo the Everly Brothers once confessed that they were so well matched musically that they often lost track of precisely where one brother’s voice ended and the other’s began. When Mr. Morricone’s compositional extravagance meets its directorial match, as it did in every one of his six collaborations with Leone as director and most of the films in Film Forum’s Morricone retrospective, it’s hard to pinpoint precisely where one artist’s contribution ends and the other begins.

“More feeling,” a sadistic Union guard tells a prison camp band forced to cover up the sound of a violent interrogation in “The Good, The Bad, and The Ugly.” In Mr. Morricone’s more than 500 films, whether the feeling begins or ends with the picture or with the melody is impossible to say.

It’s worth remembering that at the time of their release, Leone’s first Westerns, though hits with audiences, were dismissed by the American critical establishment as tasteless, cynical, lurid, and amoral reductions of what was already a dubious genre. Into the mid-1980s, Mr. Morricone produced film music at an astounding rate — he had 28 film credits in 1968 alone. And though he worked with international cinema critical darlings like Bernardo Bertolucci, Gillo Pontecorvo, and Pier Paolo Pasolini, many of the non-Leone Italian films of the 1960s and early 1970s to which Mr. Morricone contributed equally ingenious, eclectic scores were thought bottom of the barrel, if they were considered at all.

For every “Before the Revolution” (1964), “The Battle of Algiers” (1966), and “Once Upon a Time in the West” (1968), there were a half-dozen Italian, giallo horror films, like Dario Argento’s “Four Flies on Grey Velvet” (1971) and James Bond spoofs like Mario Bava’s “Danger! Diabolik” (1968). But whether he was providing the ominous military lockstep for “The Battle Of Algiers'” or keeping “Danger! Diabolik,” (a picture that plays like a 100-minute trailer for an 80-minute movie), coherent and on track, Mr. Morricone was always doing what he was supposed to do — tell a story.

In “Something To Do With Death,” Christopher Frayling’s biography of Leone, the director recalls his discomfort during a preliminary meeting with Mr. Morricone. Urged to interview the composer by his producers, Leone confessed that he found the only Morricone score that he knew to be “a watered-down version of Dmitri Tiompkin,” a prolific Ukrainian-born American film composer of the studio golden age. Mr. Morricone’s response couldn’t have been more straightforward. The producers of the films in question “commissioned me to write a watered-down version of Dmitri Tiompkin,” he explained. “A composer has to earn a living.”

But the movies are a truly modern art. All is aesthetically fair in a medium that combines technologies, theatrical styles, and storytelling approaches in a seemingly endless number of ways. Similarly, music has the capacity to cannibalize its own forms and history — recombining instruments and tempos, revisiting folk traditions, building itself out of a meticulously conducted orchestra or pulling itself apart in the frenzied ecstasy of a solo performance.

Mr. Morricone, like collaborators Leone, Mr. Argento, Bava, Mr. Bertolucci, and Brian De Palma, understands that a vital part of both music and film is the capacity to freely borrow from other sources and recombine what you already know into something new. Less satisfactory latter-day Morricone collaborations, like Roland Joffe’s moribund “The Mission” (1986), lack that sense of curiosity and discovery.

Film music, they used to say, should be like air conditioning: If you notice it, something’s not working. The triumph of Ennio Morricone’s film composing career is that his scores, as unorthodox or extravagant as they often are, almost invariably work. When a director’s ingenuity fell short of his own, Mr. Morricone filled the gap. When a director was equal to the task, as in the Leone films, the results were transcendently beautiful. It has sometimes seemed like Mr. Morricone was hired by Hollywood producers to write watered-down versions of himself. But is an air-conditioner broken or working perfectly if it’s the most interesting thing about a room?

Listen to a clip from The Mission.