When a baritone goes "cold" and forgets a line, Jonathan Khuner is there. When a soprano gets lost in her character and sings ahead of the beat, Khuner reins her in. He's the prompter at the San Francisco Opera, "the first line of defense," he says, against a singer's potential humiliation.

Khuner, 61, performs his duties in a tiny, hooded box at the foot of the stage, dead center. The audience never sees him. A Bay Area native, he was trained as an instrumentalist and conductor and started with the Opera in 1981. Since 1993, he also has been artistic and musical director of Berkeley Opera. Khuner lives in Berkeley with his wife, Jillian, an operatic soprano, and their daughters, 17 and 13.

Opera singers have a difficult job. They have to remember the words and the music and the timing, where you go onstage and what's happening with the costume. They have to be watching the conductor. They have to be giving everything to the audience. There are all these cognitive functions going on and when something goes out of whack, the words and the timing are the first things to go.

The conductor can't say anything audible (to the singer) because the conductor is 30 feet away. But the prompter is right there. So if they're singing and they forget a word, they look right down at the prompter. They see me, center stage in the footlights, and help is there. It's a very quick communication.

I signal the universal "Stop" with an upraised left hand if it isn't time for them to sing, and signal "OK" by pointing my left index finger when it's time to sing. Sometimes, I need to sing someone's part if they get lost. Or, if it's a difficult, atonal piece I might have to hum their pitches before they sing. For a couple of shows, I had a keyboard and would play their pitches for them.

You have to think like a conductor, and see how all the parts are coming together. You have to have really good rhythm. You have to know the language. And you have to know your performers because certain singers will have certain tendencies: The more you've worked with them in rehearsals, the more you can anticipate and help them stay on the beat.

The prompter's box is 34 inches from front to back and 45 inches from side to side. I enter from the floor below the stage. The first ladder I climb is 51/2 feet, and from that level, on which the hydraulic lift sits, it's another 6 feet up to the stage.

I sit in my chair. I put my music on my adjustable music stand. I've got two video monitors of the conductor and an audio monitor that's piped sound from the (orchestra) pit. I have a fan, because it gets hot in here. There's a lot of dust that comes in. If you're lucky, there are no props that come in. When people throw swords, I get nervous.

For many stage directors the prompter is a nuisance. They don't like the box because they don't want any disruption of their beautiful handiwork. But for the singers, mostly, the prompter is a great reassurance. It's not just the fact that the prompter will correct everything that's wrong. It's knowing you're going to get help if you need it. It's like a placebo effect, and it often comes just from the prompter being a close, friendly face.

I remember (mezzo-soprano) Dolores Zajick said she liked my prompting because she was never aware of it unless she needed it. In fact, I was prompting all the time. But she never consciously absorbed it except when she was aware that there was a problem.

Lotfi Mansouri, who was general director from 1988 to 2001, hates the tradition of prompters. He tried to get rid of them here. But there are singers who say, "I'm not going to sing without a prompter." This happened with Deborah Voigt. She's a wonderful singer. She came to sing a role, a role she knows very well. I was the rehearsal conductor, so I was conducting her but not prompting.

After a couple of rehearsals she said, "Why aren't you prompting me?" I said, "There is no prompter for this show." She said, "Is there anything that can be done about that?" I said, "No, nothing short of a fit by a prima donna."

She said, "Excuse me, I have a phone call to make." A couple of days later they said, "Jonathan, you'll be prompting the show."