A lot goes on between 90-plus musicians over the course of a concert. They are counting. They are following or making or changing tempo. They are listening to one another and watching one another. They are preparing for entrances, taking breaths, replacing reeds, rosining bows, draining spit, turning pages — and through all these gestures great and small, they are seeking a beauty that magically eliminates notice of all that was brought into its making. The glorious moment is there. And it is gone.

To be in the middle of the music-making is to be within a controlled chaos that can be, at different times, frustrating and inspiring, revelatory and boring, crazy and sublime. For each musician, the experience changes with any given performance or any given work. No matter how many times you've played Beethoven's Fifth Symphony, something new happens. The conductor, the audience, the weather, your hair, or who you share your stand with — these variables and many more are part of the musician's experience.

Saint Louis Symphony Orchestra Music Director David Robertson is drawn to the excitement of live performance. "What happens is that in any performance — and you see it time and again," he says, "the chorus makes a sound that causes me to realize that we can do something in a different way. I hear a certain sound from the orchestra that causes me to change very subtly what it is that we're doing. And this reaction is multiplied countless times within the space of a single second, because people react so quickly and translate it into music so quickly. The number of things happening in a second is what keeps performance very exciting."

If there's a lot going on between 90-plus musicians, the conductor is at the focal point of all that activity. "Conducting is a mysterious occupation," observes Bernard Holland, a music critic for The New York Times, "one in which the exchange of information seems to occur almost by osmosis."

Robertson's description of the musical experience — those moments he defines as "when you get into the place where music is happening" — validates Holland's idea of the "mysterious occupation."

"The distance between the sound that's coming out of the orchestra and what you do with your hands disappears," Robertson explains. "Rather than moving in air and gravity you actually feel as though you are moving sound around.

"It's not that I have the feeling that I am doing it," Robertson emphasizes. "I know that the musicians are doing it, but it's with a sense that I'm not just indicating the music. There isn't any separation between what's happening with the phrase and what's happening with the gestures."

As distance between conductor, musician, and music disappears, Robertson continues, "There is a sense of how space opens up, and in that opening up it's as though you become pulled into it ... It's like you are in a room and suddenly you push a space that has become a door, and what happens with that door is that it opens and you see something that you would not be able to see other than in performance."

As precise and practiced and finely honed as music must be, the technical aspects give way to emotional elements, which can be overwhelming. For example, in last season's performance of Mahler's Das Lied von der Erde at Carnegie Hall, Robertson recalls "the crescendo just before the moment [that mezzo-soprano Michelle DeYoung] sings 'Die liebe Erde' in the very end, there's a sense in that sound where it's as if you are accessing a power, and the power is so great that you almost have to step away from it so you don't lose yourself in it.

"There's no distance between you and the actual emotion that's being expressed, and it's not being done on a verbal level, which is why it feels like an avalanche. There's this uncontrollable thing that comes in. What's amazing is if you handle it just right, as though you are approaching this avalanche like a Zen master, you float in the midst of all of this extraordinary stuff going on."

Notes on a page, sounds being made with a bow across strings, or wind through a reed or horn, or from the beat of a drum — music conforms to those theories that describe how the flutter of a butterfly's wings may produce a storm of wind. Music creates worlds in the way the universe begins: out of nothing.

"I'll tell you what it is," Robertson says, "you have the feeling that you're touching truth, which is so big that you don't want to be annihilated by the experience. You need to keep firmly in the sense of who you are and what you are and what you're doing. Yet the thing itself can be so tremendously intense."

Eddie Silva is the publications manager for the Saint Louis Symphony Orchestra.