Imagine the scene. A snowy winter afternoon with soft sunlight streaming through the stained glass windows of an imposing Romanesque 19th-century building. The singers, more suitably dressed for football practice than choir practice, are running through Hubert Parry's setting of text taken from Psalm 122, "I Was Glad," in preparation for an ambitious Sunday afternoon concert.

In the back row, an unshaven young man with a ponytail wears a reversed baseball cap. A rotund gentleman near him has a rugby shirt hanging loosely over his belt. A woman down front is in luminous lemon leggings. She turns out to be the only one who knows the music well enough to perform without a score.

The conductor, John Rutter, is the short, bespectacled man in creased khaki twills with a Fair Isle pullover over a woolen checked shirt that's buttoned at the wrists. His accent is unmistakably "Southern English" and surprisingly penetrating for a man of such modest physical stature.

Rutter studied music at Clare College, Cambridge, in England. His first large-scale choral and orchestral work was performed when he was 24. In recent years, he has made many friends during visits to the United States to conduct workshops in English choral style and techniques. On this afternoon, he is holding an open rehearsal with Boston's Trinity Church choir.

"Let's try again," he urges, singing a few bars out loud with catchy enthusiasm. "OK? That sort of speed. We need to get that tighter. And we need a 1,000 percent concentration on these starts. They're still ragged. No slithering, please. You must feel absolutely together. And, one ...!"

The next start sounds good to most of us in the audience, and we relax—just a little—for the first time since the rehearsal began:

But that's as far as we get. The dynamo interrupts the chorus of 60 voices and the well-polished brass ensemble politely but firmly: "One moment, please, ladies and gentlemen. Sorry. We have a balance problem. The brass is overwhelming. Remember, Brass, you're accompanying. Wherever it says double forte, no more than a single forte, please."

Then, looking up at the choir: "Watch your pitch, basses. And, everyone, the diction needs to be crisper. Think of every word and its meaning. The letter D is disappearing into the woodwork."

The woodwork is looking especially warm on this afternoon of crusty snow and muted sunlight, and I'm glad to be heaped cozily in the corner of a deep wooden pew—with cushioning.

Several restarts later, the music rises exultantly to the high carved ceiling. The singers feel good. The sound is big, booming, uplifting. But as the resonance dies away, all the conductor says is, "OK, nice. Ladies and gentlemen, you've been standing a long time. Sit down."

Half-an-hour and ten strenuous voice-bursts later, the choir is still trying to be as glad as Rutter would like them to be. He turns to the organist: "More volume, please. You really can thunder and blast!" Then, spinning back to the chorus: "This is a gem of a piece. I don't want you to shoot your larynxes, but that should be a double forte. Pull out any spare bits of plumbing you've got. At the tip of the beat, please."

This time, as the tiring but determined chorus sings, Rutter's fingers go to work. The clicks are audible to me in the pews 20 rows back. His hands trace small circles in the air, and his busy fingers narrowly miss collision after collision. Then suddenly he stops and listens intently. The music flows through the aisles and caresses the tall windows, from which, in the late afternoon, all color has been drained.

No more talking, no more clicking, no more interruptions. Is that just conceivably a smile on the conductor's gleaming face—just a hint that the choir might be coming the tiniest bit closer to Parry's design?

There is visible relief in the eyes of the singers as Rutter steps down from his podium and comes forward with the warmest smile they've seen since he came in: "We hadn't met before this afternoon. I think we're going to be good friends. That was almost ... beautiful!"

For those of us who have sat with undiluted joy through three hours of rehearsal, it is the understatement of the year. Well, almost.