Alejandro González Iñárritu’s award-contending drama Birdman is full of fantastical flourishes, as its protagonist, a washed-up actor played by Michael Keaton, ricochets back and forth across the bowels of a Broadway theater where he is attempting to make his professional comeback. But there is one aspect of the actual film production that came together in such extraordinary fashion that it seems as though it could have fit in the movie as a tertiary story arc—namely the film’s percussive score, which was improvised by four-time Grammy-winning jazz musician and first-time film composer _Antonio Sanchez, using only two instruments, the drums and cymbals.

Sanchez told VF Hollywood over the phone earlier this week that he was filled with “elation, euphoria, and terror,” when Iñárritu approached him with the job offer. Sanchez had grown up listening to Iñárritu’s Mexico City radio show Magical Nights. And as the son of a movie critic and grandson of Mexican film star Ignacio López Tarso, whose 1960 supernatural drama Macario was the first Mexican film to be nominated for a foreign-language Academy Award, Sanchez had always had an interest in the medium. But Sanchez was a little hesitant.

“It is such a specific score and task that I did not know where to turn to,” Sanchez, who has a Masters in Jazz Improvisation from Boston's New England Conservatory, told us. After reading the script, Sanchez first took a more traditional approach by composing a rhythmic theme for each character. But when he sent the demo to Iñárritu, the Oscar-nominated Babel filmmaker told him his tracks were “exactly the opposite of what I want you to do.”

Sanchez visited the set to better understood what Iñárritu wanted—a score that was “very loose and organic and from the gut, very visceral and jazzy.”

But it wasn’t until Sanchez and Iñárritu convened for one untraditional, improvisational recording session in New York City that they hit upon the omnipresent “in your face” Birdman score, which shadows Keaton’s character’s internal undoing in the final film.

An accomplished improvisational musician, Sanchez knew how to improvise to the beat in his own head or with other musicians onstage. But improvising to actual images, especially those that had not even been filmed yet, was more of a challenge. So Iñárritu pulled a chair up to Sanchez’s drum kit and talked him through the movie, motioning every time that Keaton’s character would advance to the next part of the scene.

“So [Iñárritu] would be sitting in front of me with his eyes closed and all of a sudden he would raise his hand. And I would think, O.K., that means Riggan opened a door, so I would switch or do an accent or do something with the texture.’”

“We would try the scene again and then try a different kind of intensity and color . . . A lot of people think of the drums as a monochromatic instrument . . . and a lot of people do play that way but I have been experimenting with playing on the sides, the wood, on the rims, with my hands, with brushes, mallets, branches—anything to get a very wide range sonically.” He even stacked cymbals to make them sound less washy and sustained and more dry and trashy.

Iñárritu played the demos during rehearsals to make sure they worked. And they did, but he and Sanchez both agreed that the drums sounded “almost too good, too pristine” for a movie set inside an old Broadway theater. The two re-teamed in L.A., and Sanchez re-created some of his improv-ed tracks with a different drum kit that had been detuned and outfitted with vintage heads. For other tracks that were not working, Sanchez created new sounds altogether. The two also took the drums onto the street to experiment with hand-held moving microphones so that they did not have to rely on reverb, echo, and volume effects for some of the scenes in which Keaton walks through Times Square, weaving in and out of crowds alongside an actual street musician.