A show built on withholding images needs something else to carry its meaning. On The X-Files, a great deal of that work was done by sound, and in particular by Mark Snow. He scored 217 episodes of the series across the original run and the revival, and both feature films, building most of that music himself with synthesizers and samplers.
The theme is the part everyone knows. The scores are where the real craft is.
The theme
The main title music is a short, cold, whistled melody over a steady electronic pulse, with a long echo that makes each note seem to hang in the air. Snow has told the story over the years of arriving at it after several attempts that didn't satisfy Chris Carter, and finding the defining sound while experimenting with an echo effect.
Whatever the exact details, the result was unlike anything else on network television in 1993. When it was released as a single in 1996, it reached No. 2 on the UK singles chart, a remarkable result for an instrumental with no lyrics, no chorus and barely a melody.
Music as off-screen space
The scores do something more interesting than the theme. In most television, music tells you how to feel about what you're seeing. Snow's music often tells you about what you aren't seeing.
A low drone under a quiet scene suggests something in the room before the camera finds it. A slowly rising pad stretches a moment of waiting into dread. Percussive hits arrive a beat before the image gives you a reason for them. The music behaves like a second, invisible camera, aimed at the parts of the space the real one avoids.
That's an approach that suits electronic instruments. Synthesizers can sustain a tone indefinitely, shift its color slowly, and produce sounds that don't belong to any recognizable instrument. A string section always sounds like people playing strings. Snow's textures sound like the environment itself is uneasy.
Economy
There's also a practical side. A weekly series has a brutal post-production schedule, and an orchestra is expensive. Working mostly alone with electronic instruments let Snow produce a full score for each episode on deadline. But the constraint became the style. The show's sound is intimate and slightly claustrophobic, which suits a series about two people working in a basement.
The quieter work
Snow's range is easy to overlook because the dread is so memorable. The character episodes often switch to a softer palette, with simple piano lines that carry more feeling than the dialogue. Scully's cancer storyline in season four, and the show's many episodes about grief and loss, depend on that restraint.
The films let him expand. With bigger budgets, both features use fuller arrangements, but they keep the show's signature textures, so that the movies sound like the series grown larger rather than like generic studio scores.
Listening closely
Try watching a few scenes with your eyes closed. In a lot of television, you'd lose the story. In The X-Files, you often keep the mood almost entirely, because Snow put it in the soundtrack. It's one of the clearest examples in the medium of a composer who understood that his job was not to decorate the image but to complete it.