On the sprawling Universal Studios lot, inside the bungalow where John Williams wrote the music for "Saving Private Ryan," "Schindler's List" and a handful of "Star Wars" films, the composer's office is carpeted, quiet, and without a computer or screens of any kind. He works at a small drafting desk, writing notation in pencil on lined paper, within a chair swivel of a Steinway grand.

It's the piano on which Steven Spielberg hears all the main themes of his movies for the first time—and almost always the last—before Mr. Williams records the full-blown product, often at the helm of a 100-piece orchestra. Their partnership and Mr. Williams's old-fashioned artistry have endured for nearly 40 years, through Hollywood upheavals, studio shuffles and now a dramatic shift in how film music is produced and paid for. The composer remains at the top of his industry in what he describes as "a very privileged bubble."

This Christmas, two Spielberg-directed films (driven by two very different Williams scores) open within days of each other, emphasizing how closely the director's career arc has mirrored the composer's. "War Horse," about an animal that alters the lives of everyone he encounters during World War I, finds the composer in a familiar mode: orchestral overdrive, all soaring strings and thundering brass. By contrast, "The Adventures of Tintin," based on an old comic book series about a daring reporter written by the Belgian artist Hergé, features a sound that's jazzy, kinetic and playful. Co-produced by Peter Jackson and shot with 3-D performance capture technology, it's the first animated film for Mr. Williams, who says he took some inspiration from old "Tom and Jerry" cartoons he loved.

Mr. Spielberg credits the composer for making him "a better director than I could have ever been without him." Recalling the blockbuster that made them both household names, he describes the "esoteric" placeholder music he'd selected as a model for the "Jaws" score—and how Mr. Williams forced him to reconsider. "He said, "Steven, it's not an intellectual film. It's a pirate movie," the director says, adding, "John is like a great writer. He rewrites me musically every single time." After the success of "Jaws," Mr. Spielberg nicknamed Mr. Williams "Max," after Max Steiner, who wrote the first memorable original feature film score (for "King Kong") and helped establish the orchestral language of the movies.

Before pop, rock and folk music stormed into soundtracks in the late 1960s and '70s, "there was an era that harked back to the old studio system when the composer was king," says Doreen Ringer Ross, who oversees the film and TV music division of BMI, the performing rights organization. "John is pretty much the last one standing" from that time.