He Makes the Background Sing : Movies: James Horner has scored some of Hollywood’s big films, from ‘Aliens’ to ‘Legends of the Fall,’ and now: ‘Casper.’

The transparent image of Casper the Friendly Ghost may be floating across a movie screen in the background, but the atmosphere today on the Todd-AO music scoring stage isn’t exactly ethereal. Film composer James Horner, looking a decade younger than his 41 years, is trying for the third time to navigate an enormous pickup orchestra and a dozen female singers through a fast-changing, 10-minute chase sequence, filled with musical shifts that must match the image to the second.

Horner’s mood is joking but sometimes severe, especially when the orchestra members begin chatting between takes.

“I don’t think they do that with John Williams,” Horner says of the orchestra’s restlessness after work for the day is done, “but they do that with me. It’s a little bit of the age thing--I’m the same age as some of them, or younger than some of them, and don’t have the credits John does. I like to use young people because I find for the most part they’re very responsive and the mold hasn’t been set yet.”

Despite his relative youth, Horner is no stranger to the duties a film composer faces, which range from satisfying a director who may have no musical background, to producing a seamless performance from a group of hastily organized musicians, to writing the equivalent of two Richard Strauss tone poems in as little as a few days.

Since 1981, Horner has scored several of the popular films of the last two decades, including “An American Tail,” “Field of Dreams,” “Aliens,” “Cocoon,” “Glory,” “The Land Before Time,” “Star Trek II,” “Star Trek III,” “Honey, I Shrunk the Kids” and the current hit epic “Legends of the Fall.” After the Steven Spielberg-produced “Casper,” he’ll tackle Mel Gibson’s “Braveheart” and Ron Howard’s “Apollo 13.”

“I think people hire me for the slightly weird angle that I bring,” Horner says in his soft, British-tinged voice. “Part of the trick is keeping it sort of simple; you have to give the impression of not that much music playing when there’s really a lot.”

The Los Angeles-born, English-trained composer says he tried to resist the lure of the movies for a life in musical academia, until he agreed to score an AFI student film in the 1970s.

“It was like lightning,” he says. “I suddenly realized that I could be as expressive as I wanted. Each film was completely different. To me it was no different than Haydn being kept as a court composer, being paid, having the piece performed and given an orchestra.”

One of Horner’s greatest challenges recently was finding the emotional center to the sprawling, multicharacter drama “Legends of the Fall.”

“It’s a very hard story to make believable without making it seem like ‘Peyton Place,’ ” he says. “I wanted the music to cement all the things that had to be cemented without pushing it into melodrama. The tunes tend to be very simple Americana, very clean type of harmonies, nothing glitzy, very simple melodic writing, chordal writing.

“And by making some of the tunes heartbreaking, I can manipulate an audience that way rather than making it lush and over the top.”

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Although Horner cites his relationship with “Legends” and “Glory” director Ed Zwick as a model of director-composer trust, he says his own musical reference points can seem alien to many post-baby boomer filmmakers.

“I’ll work on a movie, and the director has absolutely no concept of not only where I come from but classical music. It breaks his heart when he hears Dylan sing a song, and I am completely unmoved. And they look at me like I’m a weirdo! I then try to convince them to let me do it with classical instruments. And it’s my job to get them to be as moved with my approach as they were with their song.”

Despite his three Oscar nominations and three Grammy victories, Horner is not without his critics in the industry. He is aware that some composers and industry insiders have accused him--often in letters to magazines like Film Score Monthly--of borrowing too heavily from existing concert music. It’s a charge Horner responds to with unruffled politeness.

“In the era of Mozart, Mozart was the best of the batch, but if you took any 15 composers, their music was identical to Mozart’s. If you listen to Michael Haydn or his dad, they were all speaking the same language.

“Film music is this weird, demonic thing where every score has to be absolutely different from any other score--or so the legal paper says. But if you’re an artist it’s impossible. There’s only so many ways to skin a cat. And when you actually look at ‘Casper,’ or any adventure movie I’ve done or John Williams has done--when you sort of squint your eyes, they all sound the same. Maybe one’s better crafted than the other or more subtle, but when you kind of look at the overview, they all sound the same.

“I find classical music--serious music--a wondrous world, and I draw from it.”

Horner admits that earlier in his career, the remarks of his peers--and his place in the industry pecking order--made him an active Hollywood player, until he re-prioritized after the births of his two daughters and his move with them and his wife, Sarah, to Calabasas.

“Now I try not to keep abreast of what people think. In my heart of hearts, if I’ve done the job and it brings tears to a director’s eyes-- good tears, I mean!--when we sit through a screening and I cry, I know I’ve nailed it.”

These days, Horner is abuzz with ideas for both “Braveheart” and “Apollo 13.” The latter, a chronicle of the near-disastrous 1970 space flight, poses a particular challenge: “If you start off with a big score, it sets an audience up for just another sci-fi movie, except this one is a documentary; you know where it’s going to end.

“What I’m trying to get out of the story is the idealism, everything that was great in the guys at Mission Control and in the capsule, the best thing about NASA. And that’s a very elusive thing to bring out with a flute, but that’s what I want--idealism, in a very quiet way. If I go in with something you don’t expect at all, it’ll be just magical.”

Of course, there’s another built-in challenge: making the deadline, especially since “Apollo” has been moved up from a November release to July 4. “It’s a typical story,” the composer says with a sigh. “I have to do what I was going to do in 10 weeks in four.”

Of his craft, he says, “My trick is that the films are all so different. I have no high ambitions to win 35 Academy Awards. I just try to be the best at what I can be and work on the best movies I can and not get too wrapped up in the day-to-day ups and downs of it, which is difficult enough.”

His quiet voice rolls into a laugh. “I’m just serene on the surface!”