AT 11:02 ON a Friday morning not long ago, Joan Dornemann climbed up five iron rungs to sit in a pint-sized, makeshift-looking box at the edge of the huge stage of the Metropolitan Opera. She was ready of prompt.

According to Webster's Seventh New Collegiate Dictionary, to prompt is "to assist one acting or reciting by suggesting the next words of something forgotten or imperfectly learned." That is not, however, what prompters at the Metropolitan Opera House say about their work.

"You can't prompt someone who does not know the music," Dornemann said during a break in the final dress rehearsal of Verdi's Luisa Miller, which will be simulacst live Saturday night at 8 o'clock on Channel 26 and WETA-FM."People say, and it bothers me, 'oh, hmph, he needs a prompter!' as if that meant that he does not know the musci well enough. That is not the case at all.

"The problem is that everything has to happen by very intense coordinaton. And it must all be expressive so that the music will come out the way it should.... Dornemann stopped for a moment as if thinking of some of the problems in "Luisa Miller."

"But in an opera like this," she went on, "the words are very archaic -- by Caamarano -- and there are patterns. If the singer has to repeat a passage, as so often happens, and the words are different the second time, and he has to thing, 'Now is this "D'all aule raggianti," or is it "La dove sorgea?" the dramatic impulse can be lost. So the use of the prompter can keep the singers from counting with their fingers, 7, 6, 5, 4, CUE!...

"But there are about nine million things to think of. I learned lots from singers in Barcelona where I started as prompter. Half of it is the words, and half the music. The concerted passages are the most difficult. Labo (Flaviano Labo, a tenor formerly with the Met) said to me once, about the big ensemble in "Aida," 'You don't make a move unless it means something. Or I will sing.!"'

Dornemann's specific meaning became clearer as she went on, "The prompter is another set of hands for the conductor. Dornemann, "by using gestures," conducts throughout a performance, but with very different motions from the conductor, since she is working for a different purpose. She gives her signals to each one of the principals and to the chorus.

"Prompters are treated so nicley by the singers," she said. "They are not seen by the public, which makes my mother say, 'Oh, and you work so hard!'

"Because the concerted passages are so hard, I work with records and with props like salt shakers! Why? Because I need to move things around the way the singers will move."

A look at the tiny space in which prompters work makes their accomplishments all the more amazing. The minuscule cubicle is set dead center of stage, right at the edge, behind and above the orchestra, and directly in line with the conductor. At the Met you get into it by climbing up four very narrow iron steps. On the top one, if you're smart, you turn yourself around and hosit yourself up the last step backwards into the seat, which is a kind of grown-up high chair. At that point your head is right up against the to pof the box, and you are staring at the huge stage, the floor of which is level with your eyes.

Two miniature TV screens are there, one on either side of the misci stand on which the prompter's score rests, with its thousand deatiled notes and reminders. The TV screens give vision to right or left, depending on where the crucial action is going on, while keeping the conductor and his directions in the rear-view mirror. The score is placed high enough that fast look at it can be snatched while the singers are kept in sight.

At left there is a phone with which at prompter can call any where in the house in case of emergency -- say a riped costume, fainting soprano, or missing prop. All of these are so placed that the prompter is always facing up. If a little less tension would help at times, there is a metal bar on which you can rest your feet.

Since prompters are rarely seen except by the cast, the backstage personnel and the orchestra, what they wear is a monor matter. But Dornemann syas that after one out-of-town performance where she was requested to wear a dress, she realized the greater wisdom of pants suits.

"The prompter is always a touch ahead," she noted. "I can see a page of music at a time and memorize. Then I am free to look at the singers. You NEVER frown! Oh, maybe you look -- " she stopped and made an indescribable look partly sorrow, partly amazement, as she said, reflectively, "Oh, did we really mean to do THAT?"

What does she do if singers go flat or sharp?

"We have hand signals, sort of universal sings that point the way that they should go.

"It's very lonely out there," Dornemann continued. It is also very busy. As Verdi's music rolled along, Dornemann was constantly cuins, her hands moving differently from the conductor's as she gave the chorus a cue for which the conductor had no free hand.

And how do conductors like prompters?"It depends," Dornemann said. "The conductor may catch this bit and I that. When the coductor is someone you know and are comfortable" -- there was a pause -- "well, some conductors ask for certain prompters."

And how about those times when th prompter's vocie can be heard, especially over the air, but also in the house?

"The shell behind the prompter's box usually keeps the voice from being heard out in the house at the same time that it projects your vocie toward the singers. But one of the radio mikes is right next to the box," which is why listeners often hear the prompter over the air.

"The recitative passages are the hardest for this, because there is n accompaniment and you can be heard so easily.

"You form the words and sounds with your lips," she added, disclosing her professional secrets. "You do it seconds ahead, depending on the score and the tempo; and very expressively if the music is expressive at that point. You must make the words go the way their meaning and the music go." Illustrating with a line from Massenet's "Werther," she sadi, "If you come out with 'Ecoutez-bien," in drill sergeant manner, you will get a brusque response from the singers. You sing very little. On occasion if there is a hard pitch, I may try to intone it."

Veteran baritone Cornell MacNeil was talking about Dornemann's hand signals when he rated prompters: "It depends on who the prompter is. I pay a lot of attention to Jan because there are musical things we do."

Dornemann went on. "Then eye cues are very important. Caballe has a kind of dropped eyelid gesture she makes when she wants help."

What Dornemann said next seemed strange at first, until she explained it. "The lat act is always dangerous. So is the next phrase after a big high note, or the end of a big scene. Because you have put so much energy into that note, that phrase, that scene, and things can go wrong in what comes right after. And by the last act, there is the danger that you might think, 'Good, this is the last act,' and be tempted to let down."

Dornemann is also an assistant conductor at the Metropolitan, but she may sever as prompter as often as four times a week. Surprisingly, after her description of the host of deatils that go into her work, Dornemann said, "Sometimes you simply put your hands down and do not move. That can occur in a passage with very precise rhythms which only the conductor must do."

Suddenly, at 12:41 p.m., toward the end of Act Two of "Luisa Miller," the whole rehearsal stopped. The lights went out on stage, and Stage Manager Christopher Mahan joined Stage Director Nathaniel Merrill on the stage with the entire chorus.

Dornemann had climbed up into her prompter's box at 11:02 that Friday. She had helped pilot the rehearsal safely through the difficult passages, and now she could climb down, a bit stiff here and there, out of the musical crow's nest.