Andrea Mandell

USA TODAY

Curious how director Damien Chazelle managed a troupe of dancers, a Los Angeles highway ramp full of cars and a complicated musical number at the start of La La Land?

It came together on his iPhone.

In this exclusive video, watch Chazelle, 32, track the original choreography from Another Day of Sun in a parking lot, the genesis of what would become a stunning, six-minute introduction to his modern musical, which ultimately scored a record-tying 14 Oscar nominations.

Plans for the scene began with Chazelle and choreographer Mandy Moore "walking it through, talking it out and diagramming it out. The dance and the camera work — it was sort of a chicken-and-egg problem; they had to be formulated at the same time," he says.

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Without knowing which highway they'd be working on (or how many lanes it would have), Chazelle began mapping out camera angles with his director of photography, Linus Sandgren, as Moore worked with a skeleton crew of dancers, sending the director pieces inspired by the athletic choreography from Seven Brides for Seven Brothers, the opening of Jacques Demy's The Young Girls of Rochefort and West Side Story.

Even "casting" the dozens of cars took time, says Chazelle, with the team trying to ascertain "what kind of cars could support dancing without caving in, how to reinforce the cars, how to make them safe."

It was only then the production team moved outside with a partial cast, "doing what I call ‘full parking lot rehearsals,' " says Chazelle, with a few cars and props standing in for the median. In the video, that's "me running around with an iPhone basically doing the number," he says.

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That video "became sort of our bible" when La La Land shut down the actual highway ramp for the first time in Los Angeles, says Chazelle. Except the rehearsal was kind of a wreck.

"We were able to secure the highway location for half a day a few weeks before shooting," the director recalls. "We all went in feeling pretty confident because we were quite happy with how the iPhone (video) had worked out. And the dress rehearsal was a total disaster. The crane could not move the way an iPhone moves. … The choreography looked different, the slant of the freeway created a lot of other challenges. Everything was just tricky. We had to go back and regroup and make some adjustments."

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Several weeks later, in August 2015, it was go time for Another Day of Sun, with Emma Stone and Ryan Gosling sitting in their respective cars on a ramp connecting L.A.'s 105 and 110 freeways, waiting for stubborn clouds to move.

"We went back to the location, a little chastened, a little humbled and a little nervous," says Chazelle. "And then we shot the real thing, which you see in the movie."