“Could the vibes and orchestra bells play softer, please, at bar 48? Then at 52, we come back to our previous dynamic, OK?”

Composer Alexandre Desplat is in the middle of his fourth day on the Sony scoring stage, conducting a 75-piece orchestra to perform his delicate, charming music for “The Curious Case of Benjamin Button,” Paramount’s long-awaited adaptation of an obscure F. Scott Fitzgerald story about a man who ages in reverse and the complications that result when he falls in love with a 30-year-old woman.

The trailer for the Christmas release, which stars Brad Pitt and Cate Blanchett, has generated a fair amount of buzz for its dreamy sense of wonder, accentuated by temp music from Saint-Saens’ “Carnival of the Animals.” It’s in this vein that Desplat rehearses cue 2M1 — the four-minute “Never Know What’s Coming” — with director David Fincher listening intently while watching how the scene plays with the music on a monitor in the booth.

The music is in waltz time, the textures light and feathery. The addition of celeste and harps lends an almost magical feel, and the 50 strings give it a rich overall sound. Desplat’s gestures are as careful as his orchestration — he conducts without a baton, lifting his hands only slightly from phrase to phrase. Striving for perfection, he records multiple takes and is finally satisfied nearly two hours later.

During a break, Fincher explains why he hired the French-born Oscar nominee (“The Queen”) and Golden Globe winner (“The Painted Veil”): ” ‘Birth’ is one of my favorite scores, as good as anything I’ve heard in 25 years,” he says. “Alexandre’s music is playful and witty. But I think it’s the oblique nature by which he goes at things. With composers, I think you have to hire somebody you believe in and then get the fuck out of the way.”

The musical demands of “Benjamin Button” are even more stringent than usual, because the composer must not only deal with dialogue but also voiceover narration throughout the film. “I don’t want to hear and see the same thing,” Fincher says. “The music never needs to do what the picture’s doing.”

Back in the big room at Sony, Desplat makes subtle changes in cue 10M3, “Final Regression,” substituting a blend of tenor saxophone and alto flute for an alto sax solo. “Gentle and elegant,” the composer suggests to the players. Fincher gives a thumbs-up from the booth. Another good take, and the orchestra applauds.

“I wanted the music to be like a chamber ensemble,” Desplat says a few weeks later via phone from Paris. “The story of Benjamin Button is somehow epic, but the orchestra is not huge and loud. Chamber music is the most delicate thing you can imagine. Precision on such a score is crucial because if the trumpet doesn’t have the right mute, doesn’t play perfectly in tune or plays too loud — it’s very exposed. It doesn’t work.

“It’s a tricky movie,” he adds, “a great challenge. Some movies are difficult to score and there is no pleasure. This was a lot of pleasure, but difficult because I had to work twice as hard.”

He wrote 80 minutes of music but laughs as he points out, “for an American movie, it’s not so much.”