Horner — the 38-year veteran responsible for the original scores of "Titanic" (for which he won an Oscar), "Aliens, "Field of Dreams," "Braveheart," "Apollo 13," "Avatar," and countless others — would create the original sound and Franglen would be his second-in-command, responsible for overseeing production.
The two had been teaming up together since "Titanic" in 1997, and for over a decade Franglen had become an integral part of how Horner works.
"We were a great team," Franglen recently told Business Insider. "It became evident that we loved working together."
But that all suddenly ended on June 22, 2015, when Horner, while flying a single-turboprop plane by himself, was killed when the plane crashed in the Los Padres National Forest in Southern California. He was 61 years old.
"I have this text from him the night before he died and I had spoken to him earlier and he was in a great place," Franglen said. "Then the next morning there was this stream of texts and the phone calls started coming in — I would trade anything not to have this discussion."
At the time of Horner's death, he and Franglen had a full plate of projects.
Horner had signed on to do the scores for the Matt Damon thriller "The Great Wall" and the Mel Gibson-directed "Hacksaw Ridge," as well as for "Avatar" parts two, three, and four.
Then there was the project that needed their immediate attention, a remake of the 1960 classic Western "The Magnificent Seven" with Antoine Faqua directing and starring Denzel Washington and Chris Pratt.
Not only did Horner and Franglen have to fit the movie into a tight schedule, but they also had to go up against history. The original movie has what some might call an irreplaceable score written by Elmer Bernstein, a legend in his own right.
"Like anyone else in this business, I have Elmer in my DNA, and [James and I] talked at length about how could we reference it, how do we make sure that the audience knows we understand the heritage of the Bernstein score," Franglen said.
He and Horner thought they had cracked the code when Horner died.
When news hit of Horner's passing producers for projects like "The Great Wall" and "Hacksaw Ridge" began a search for new composers, but Franglen didn't want to let go of "The Magnificent Seven."
Franglen flew to Los Angeles and gathered Horner's team of music editors and lead orchestra conductors to create Horner's final score.
"I said to them, 'I don't want these things to just disappear, I want to at least play them for Antoine,'" Franglen said.
Franglen and company hired an orchestra and recorded Horner's ideas for the "Magnificent Seven" score. Franglen then flew to the set of the movie in Louisiana to play the music personally to Faqua.
"I said, 'Look, I have a gift from James, this is how I think he would like the score to have sounded and I just wanted to give it to you,'" Franglen said. "And it was emotional for us all. He sat down and played it and he listened and at the end he said, 'Let's do this. I want you guys to finish this score.'"
Franglen led the completion of the score, which took nine months and included the creation of some new pieces outside of Horner's work to have enough music for the entire movie.
The finished product is very distant from the Bernstein music in the original (which plays over the end credits in the new movie). Though at times it has the big orchestra sound like the original, there's a darker tone that mirrors Faqua's film.
And though everyone knew this would be the final score of a legendary composer, Franglen didn't want it to sound that way.
"The last thing we wanted to do was make this a mausoleum," he said. "It was meant to be a film score for a very specific film. Antoine needed music that reflected his film and that's the first job of a film composer, to serve the film and be the emotional heart. James would have been adamant that the score has to serve the film."
Franglen has recently completed the score for Terrence Malick's documentary "Voyage of Time: Life's Journey," and he hopes to continue in the footsteps of his mentor. Looking back, the experience he got from working alongside Horner for years is priceless. But the great loss remains: He is no longer alongside his friend.
"He was one of my closest friends," Franglen said. "It was a point where if he called the house and my son picked up the phone they would just chat away. We considered him part of the family. It was a horrendous loss."
"The Magnificent Seven" opens in theaters on Friday.
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