James Horner’s scores set the emotional tone for hundreds of films, but his legacy will be inexorably tied to one director, with whom he shared friendship, accolades, and admiration for more

than 30 years. “It’s sad and shocking, and it’s such a waste in a way,” James Cameron tells PEOPLE. “There’s so much music he could have done, and we were looking forward to our next gig

together.” At the time of Horner’s death from a plane crash on Monday, he was signed on to write the score for Cameron’s next three _Avatar_ movies, and as Cameron spoke to PEOPLE, he said

he was looking at a note on his desk reminding him to call the composer about getting started. While he never began work on the project, _Avatar 2_ remains listed as his final credit, making

out, but everybody knew everybody in that little group of guerilla filmmakers,” he says. After Cameron and the crew saw the finished movie, he says, “We were stunned at the score – this big

orchestral score that sounded like something like a $20 million movie and this was a $2 million low-budget space opera.” Six years later, the director earned his studio debut with another

sci-fi flick called _Aliens_, and tapped his old colleague to write the music. “It was the first film where we could afford an orchestral score and James was on the top of our list to do it.

He was kind of the hometown boy made good from our group back then.” However, despite the mutual respect between the composer and director, their working relationship on the horror flick

wasn’t hitting the right notes. “It was rocky at first on _Aliens_,” Cameron admits. “I don’t think it was a good experience for either one of us, and I thought, ‘I’m never working with this

guy again.'” Cameron chalks up the turmoil to Horner’s heavy workload, explaining that “his success was sort of catching up with him in a way, and he kind of left us with a pile of

music and we had to sort it out ourselves.” It would be another 10 years before the two worked together again. Horner went on to score award-winning films like _Apollo 13_ and _Braveheart_,

while Cameron sealed his reputation directing blockbusters like _The Abyss_, _Terminator 2: Judgment Day_ and _True Lies_. During their hiatus as partners, Cameron says he never stopped

admiring Horner’s work, and hoped the two could eventually find a way to work together again. When they finally reconciled on _Titanic_, Cameron says “we were so overly accommodating to one

another at the meeting, we joked about it for years afterwards. We actually worked out a way that we could prevent the problems that had happened before.” Horner ended up crafting three

themes that he wrote based solely on multiple readings of the script and watching more than 30 hours of raw, uncut footage. “I literally teared up at each one, and it was just him at his

piano, by himself, no technical people around or anything and I knew it was going to be a fantastic score at that point,” Cameron says. _Titanic_ cemented Cameron and Horner’s close

friendship, and earned the pair their first Academy Awards, for Best Director and Best Picture, and Best Score and Best Original Song, respectively. After the film, both composer and

director slowed their careers in film and focused on other passions. Horner turned his sights to the sky, while Cameron looked to the depths of the ocean for adventure. “I think some people

are satisfied living vicarious adventures through film, and I think he and I were attracted to real adventure,” Cameron explains. Horner became an expert pilot specializing in the dangerous

art of aerial acrobatics. At the same time, Cameron set about exploring the ocean bottom, building custom submarines and eventually breaking the world record for deepest solo dive in 2012.

“It’s about testing yourself against a very rigorous and disciplined kind of challenge, or using your own rigor and discipline against a challenge. In his case, it was a pretty unforgiving

area of aviation in aerobatics, and for me, it was the most unforgiving sort of realm of technology and engineering in the world, which is the deep ocean,” Cameron notes. While his career

was cut tragically short, Cameron ranks Horner among film’s greatest composers. “I think he’s up there with John Williams and Hans Zimmer, firmly implanted at the top of the firmament of the

current great masters.”