Virtuous Ellen Chandler is browsing through the blouses at the Boutique at Pine Valley when that vicious homewrecker Cynthia Cortlandt walks in.

"Yeccchh!" shouts Ellen, retching.

This scene will not, repeat not, be part of today's episode of "All My Children." It's only 9 a.m. in the show's West Side studio, the end of what's called the dry rehearsal. The actors are in sweat shirts and curlers and no makeup and feel free to horse around. Director Jack Coffey, who later will virtually sprint through scenes, is still moving at a brisk walk. The camera operators are still on their first or second styrofoam cups of coffee.

As soap opera episodes go, Coffey says, this one will be fairly routine. No big crowd scenes, like Tad and Hillary's wedding last week. No remotes, like the fox hunt viewers will see next week. Just the usual 42 minutes and 25 seconds of life in Pine Valley, the imaginary New York suburb with a per capita murder rate higher than Detroit's and more marriages and divorces than Reno.

There's the Erica-Jeremy-Natalie triangle. "Erica has just been told, though she doesn't believe it, that Jeremy slept with Natalie, schemer that she is," recounts Coffey, running through the plot developments of show No. 4,186, being videotaped in early February, to be aired today (1 p.m. on Channel 7.)

There's Benny and Donna and Nina and Cliff. "Now Benny, he recognizes his dilemma -- that he loves Nina. What's he going to do about it?" This is a problem. Benny, chauffeur to the wealthy Cortlandts, is married to Donna, the reformed hooker who now runs the Glamorama beauty salon. Nina is married to nice young Dr. Cliff Warner, but is currently immune to his charms, as she's suffering from amnesia.

"And Cynthia has returned," Coffey adds ominously, as though the pot isn't boiling hard enough already. "She's another one of the bitches, Palmer Cortlandt's ex-wife. She's back to cause trouble. What trouble, we don't know yet."

People no longer need to tune in to daytime dramas for convoluted tales like this, of course. Prime-time television is full of soaps and soapy mini-series, and the suds are creeping inexorably into shows like "Cagney & Lacey" and "Hill Street Blues." But there are differences.

Producing a week's worth of "All My Children" costs about $550,000 for all five shows; the "Dynasty" budget, to take another ABC drama, is well over a million dollars an episode. An hour of "All My Children" is taped in one day, a tenth the time required to film an hour of "Dynasty." Soaps don't spend much money, comparatively speaking, but they pull in plenty. Daytime programming accounts for the majority of the network's profits. "We're the breadwinners," says Jacqueline Babbin, "All My Children's" producer.

Like most breadwinners, her team works its collective tail off. All last night crews tore down sets and hammered up new ones; the lighting people reported for work at 3 a.m.; actors began arriving this morning at 7 and won't finish the day's taping until nearly 12 hours later. It takes about 200 people to put "All My Children" on the air, and they have put it on the air every weekday, including Christmas and the Fourth of July, 52 weeks a year, without a hiatus or a rerun, for 16 years. Hoary as the genre sometimes seems, with its ancient plot devices and molasses pace, the sheer volume of programming and the speed of its assembly seem to astonish even the people who manufacture it.

James Mitchell, who's been playing Palmer Cortlandt (the ruthless industrialist) for years, still marvels at the phenomenon. "The logistics of getting the right sets up, the calls in, the extras hired, all the mechanics of getting it ready for 7 in the morning," he muses. "And then, the skills of a couple of hundred people to get it on tape by 7 at night. It's amazing."

Long before today's marathon of rehearsal and taping, of course, writers, producers and ABC brass were considering precisely the same questions as viewers:

Just what would Benny do about the amnesiac Nina?

Could Natalie really come between the much-married Erica (recently rescued from an avalanche) and her lover Jeremy, released at last from the vows of celibacy he took when he joined a Tibetan monastery?

What's to come of Angela's sexual harassment case against Dr. Voight, and of Greg's troubling infatuation with Robin (all teeveeland grieved when Greg's young bride Jenny got killed by a bomb-rigged jet ski in 1984, but it's hard to rejoice about his new romance when Robin is so plainly Up to No Good).

And can Tad the Cad -- previously famed for sleeping with the town slut and her mother, though not simultaneously -- really settle down with the terribly sweet Hillary?

Characters' long-term fates are decided at monthly story meetings, when the three head writers, producer Babbin and ABC's daytime programming chief sketch out plots for the next several months. Last month, recalls Wisner Washam (hired in 1971 by the show's creator, Soap Queen Agnes Nixon herself, and now the longest tenured head writer in daytime), a dual-personality idea was shot down because another soap had already embarked on one. But the brainstorming that followed produced the Erica-Jeremy-Natalie entanglement and Brooke's new career. These people know what's going to happen to Nina and Cliff.

After the meeting, Washam and the three other in-house writers produce rough drafts of the story lines, which then become 12-page descriptions of each show's events, which five dialogists eventually turn into scripts -- cranking out one a week for at least $1,500, the Writers Guild minimum. Meanwhile, the daily details appear in tiny handwriting on the calendar dubbed "Wisner's Squares": which eight or nine of the show's 100 sets will be needed, which actors will appear, who will get injured in the fox hunt. The squares also remind the writers about more prosaic matters -- that Jean LeClerc (a k a Jeremy) will be performing at the Williamstown Theater Festival this summer ("Jeremy will have to take a trip back to Tibet," LeClerc suggests); that someone's contract expires soon (maybe that's who'll get injured at the hunt).

But there's still plenty of room for sudden inspiration. A few weeks before Tad and Hillary's wedding, which everyone agreed could not possibly proceed smoothly, the writers came up with the novel idea of having Tad lock himself in the vault of the Pine Valley Bank at the moment he was supposed to be at the church. And of having Hillary run out of the church in search of her wayward groom and appear to be hit by a car.

"Straight out of 'The Perils of Pauline,' " says Washam with satisfaction. "And there's the interesting matter of who was driving the car, by sheer coincidence . . ."

The writers get to see how their handiwork plays each lunch hour when, sending out for sandwiches, they cluster around the conference room TV. Like roughly 9 million other Americans, they're hooked on "All My Children."

The actors, on the other hand, virtually never see the people who distribute the amnesia and fatal accidents. Fraternization between actors and writers, whose offices aren't even in the same building, is a soap culture taboo. "Archvillains on our show are actually extremely nice people," explains writer Lorraine Broderick. "You want to keep the character clear in your mind."

Besides, soap tradition decrees that the cast never know the endings of the show's various sagas. Even Cliff and Nina don't know what will eventually happen to Cliff and Nina.

"It's like a serial novel," comments Kate Collins, a k a Natalie. "I got hooked on my own story. Whenever they give me the next script, I run into a corner and read it to find out what's happening."

By the time blocking begins at 10, the pace is already picking up. The studio is two stories high, warehouse-like, a wide center aisle edged with sets on both sides. Director Coffey and three cameras are cruising up and down the aisle, running swiftly through the episode's 27 scenes, a ceilingful of stage lights briefly illuminating Erica's living room, then Palmer's library, then Cliff's office. Actors appear when paged, diffidently recite two minutes of dialogue, then wander off.

Coffey -- one of four directors who take turns doing the show -- has already planned all the shots and staging; now he peers over half-spectacles into a monitor. "Door is open, Donna's in the bedroom. Low shot of his eyes . . . " he instructs the camera operators and the technical director in the control room. He knows when Donna should cozy up and when Benny should cross the room (and he usually does use those names instead of the actors'). "Aaaand, kees kees," he cues, as Donna and Benny go into the clinch. "And he goes over her shoulder . . . Aaaand five, four . . . "

Now Benny and Donna's apartment goes dark, and it's off to the Boutique. "Where are you going?" Coffey demands as Jeremy exits through the wrong door. "I've never been in here before," Jeremy apologizes. "Aaand, we cue the ever-sneaky Natalie . . ." Coffey picks up. "Okay, camera 2."

Dr. Cliff -- whose real name is Peter Bergman and who does a cough syrup commercial in which he announces that he's not a doctor but he plays a doctor on TV -- finds a folding chair between scenes. The two-hour blocking, he explains, "is not for actors. It's for lighting people, for finding boom shadows and taping the floor. It's about cameramen finding out if they're getting pictures of other cameras instead of us."

Like other soap stars, Bergman/Cliff alternates between pride in meeting the considerable demands of the genre and a nagging sense of not being taken entirely seriously. "I'm told in film you do half a page of dialogue a day," he says. "In nighttime, three pages is a pretty full day. I have 25 pages today. I've had 40."

Doing 40 pages a day, however, only rarely brings in movie deals, Broadway leads or sheer celebrity that lures people into show business in the first place. There are famous people who were once on soaps, but being on soaps rarely makes one truly famous. Only two of "All My Children's" original cast, Ruth Warrick (who plays Phoebe) and Susan Lucci (the highest paid star in daytime, who plays Erica), are still with the show, but of the dozens who've left, none has reached household-word status. "It's not what it once was, the little bastard," Bergman says of the soap stigma. "But it's still there, a little snobbery . . . Agents kind of say, 'You don't want to stay on a soap too long.' "

He has been on this one for six years.

After lunch, the run-through. Everyone's coiffed and costumed now, Nina and Donna in their respective filmy negligees, Erica in her silk Ungaro, Jeremy properly Bronzetoned by a makeup man.

Coffey orchestrates from the control room, waving a blackboard pointer. It contributes to the dramatic effect and, more pragmatically, shows the technical director by his side when to throw switches for camera changes. Babbin, producer di tutti producers, takes a swivel chair at the back of the darkened room and props her Wallabees on the table.

Babbin's job is to mutter. She mutters about why Brooke's whistling teakettle suddenly stops whistling before Brooke has turned it off. The sound-effects room upstairs, where a harried engineer slams tapes of doorbells, honking cars and the occasional slap into a cartridge machine, hears about that one. She mutters about the prim white-collared dress Nina wears in one scene. Fashion designer Carol Luiken, on hand for the run-through, hears about that one. "She looks like a 3-year-old schoolgirl," Babbin complains. Luiken protests that the frock is "classic," but by the final taping, Nina is wearing something else.

A soap's producer, Babbin explains, is "responsible for every single bloody thing that happens on the show." She reads and red-pencils scripts, scouts fox hunt locations (Westbury, Long Island, was the choice), casts roles. Babbin has worked in nighttime, too (as ABC's vice president for mini-series she oversaw "Masada" and "The Winds of War"), and she summarizes the difference in a word.

Money.

"Hell or high water, we're doing a show a day. You make a choice: 'Am I making the scene that much better, at the expense of a tired cast and crew?' We'll settle for something not as perfect as I wish it could be. You can't be as much of a perfectionist as you would like on a soap; that's a fact of life."

It's also a fact that ABC is growing edgy about its once-indomitable daytime bloc. "General Hospital" and "All My Children" helped keep the network No. 1 in daytime for six consecutive years, a title it lost two years ago to CBS (which has stronger morning game shows) and has yet to regain. "General Hospital" still leads daytime ratings, but "All My Children" is no longer the perennial No. 2; it generally ranks third behind "The Price Is Right," sometimes slipping to fourth behind "The Young and the Restless," CBS' surging soap.

While that still makes "All My Children" a ratings success, there are portents of declining interest. Soap Opera Digest voted it Most Disappointing Show of the Year for 1985. "The story lines have gotten very tired," says Digest Editor Meredith Brown, who notes that newsstand sales of issues with "All My Children" stars on the cover have dropped off. Nor are the show and its cast faring well in Daytime TV's monthly readers' polls.

Add to this the general jitters caused by ABC's sale to Capital Cities Communications Inc. -- which has produced personnel changes, layoffs and severely reduced Christmas partying -- and it's not surprising that certain cherished soap traditions are yielding to slow change.

Glacial as "All My Children's" pace seems compared with any prime-time series, it's considerably faster than it used to be. In 1977, when the show first changed from half an hour to an hour, an episode had 17 scenes; now the usual script calls for 24 or 25.

"We used to have secrets, really surprise the audience," writer Washam recalls. Now, not only do fanzine columnists reveal story lines (the writers suspect paid inside tipsters), but ABC's own promos let various cats out of bags. "They'll show a scene a week in advance," says Washam, who disapproves.

In other ways, though, this remains a staunchly traditional soap. While "General Hospital" once served up a plot involving a megalomaniac bent on controlling interplanetary energy sources, "All My Children" generally steers clear of sci-fi mayhem. True, one of Erica's lovers' father was a Nazi. But viewers don't tune in to "All My Children" for cops and robbers; they want to know what's going to happen to Cliff and Nina?

Even though the show has a composer whose score calls for an orchestra, you can still practically hear that haunting organ chord at the end of each scene.

"Shall we go get some abuse?" Benny asks Donna in the makeup room. "You know how I love it," she purrs. It's 3:30, time for the cast to assemble on the set for "notes." Translation: The director tells the actors what they did wrong in the run-through and how to improve for the final taping, half an hour away.

"It's getting a little shrill," Coffey tells Cliff, who's understandably upset that his wife is rejecting him for a chauffeur.

"We could use a little more guilt at the beginning of this" is the advice to Benny, whose scenes today include a fantasy of Nina wearing something sheer and red and beckoning longingly from her satiny bed.

As for Cynthia, back in action for the first time in five months, Coffey suggests she act friendly. "She's trying to find a way back in," he says, "so she can embarrass the hell out of Palmer again."

"Can I have my wheelchair, please," Cynthia groans when it's time for final primping and taping. There are open yawns around the set; people who've been hanging around the studio for at least eight hours now have to stoke their adrenaline and perform. Jeremy is popping chocolate; Cynthia grabs a Coke.

Somehow, when Coffey grabs his pointer and announces, "Roll and record," it all gels. Benny sighs; Natalie flounces; Palmer harrumphs and Nina flutters. It's not the Royal Shakespeare Company, but people supposed to look angry look angry. Despite the inevitable fluffs and retakes -- a boom squeaks, Cynthia breaks into expletives when a door sticks -- the taping is over by 6 p.m. In the can.

"No weddings, no avalanches; an easy show," Babbin concludes.

The trucks idling outside the studio can begin unloading now, delivering tomorrow's sets. Working through the night, the set crews will disassemble Palmer's office and the Boutique at Pine Valley, Nina's shiny boudoir and Brooke's living room, sending them back to be stored at a 34th Street warehouse. They'll set up six new sets, including a Pine Valley Hospital room (the fox hunt accident victim will need one), the camera shop (wherein Robin bamboozles the trusting Greg) and a hospital drug room (hmmmm).

They'll be mostly finished by 3 a.m., when the lighting specialists arrive. And dawn will break over Pine Valley once more.