LOS ANGELES -- At the beginning, even the father of the show's tiny saviors, Ashley and Mary Kate Olsen, did not think the ABC sitcom "Full House" was very good. "I didn't think it was going to make it," Dave Olsen said.

The first reviews ranged from hostile to tepid, the kindest saying at least the show aimed to please. One network publicist remembers "a press conference when the show was launched that was just a disaster -- everybody hated everything."

So why, deep into its fourth season, is the 8 p.m. Friday program so firmly locked into the schedule, usually winning its time slot and often hitting the national Top 10? Why do I, a man in my forties of intellectual pretensions, wait eagerly at 7:59 p.m. each Friday for the familiar Golden Gate Bridge shot that heralds the latest bit of overcute "Full House" dialogue from one of two 4 1/2-year-old little girls?

Why am I writing this piece in the first place?

Perhaps it's witchcraft, preschool style. Perhaps it's the triumph of an entertainment industry that has created a subculture of child actors, teachers, handlers and writers, expertly designed to give a generation besotted with parenthood just the kind of feedback their own children can muster only once or twice a year.

Like much of the viewing public, my family and I are hopelessly infatuated with the Olsen twins. The two blond, big-eyed girls, in most ways unremarkable members of their generation, live in a little three-bedroom house in the San Fernando Valley, tease their older brother and younger sister, play ferociously in the mud whenever allowed and, through psychic forces imperfectly understood, now rank just below Bill Cosby as the most likable people in America.

Steven Levitt, president of Marketing Evaluations/TVQ, the Port Washington, N.Y., measurer of national popularity, gives the twins an appeal rating of 48, just below Cosby's 53 and above Steven Spielberg's 45. What is more remarkable, Levitt said, is that the girls have managed such a score when only 49 percent of people surveyed recognized them, compared to 92 percent for Cosby and 76 percent for Spielberg.

Child actors as raw and unpolished as the Olsen twins have been a staple of television and film since the "Our Gang" comedies, and earlier. There was nothing unusual about fledgling "Full House" executive producer Jeff Franklin's wish in 1987 to populate his new comedy about three men raising a family with children that would in some way catch the eye.

He found two very bright and competent actresses, Candace Cameron and Jodie Sweetin, to play the two older sisters. Then, he said, "we got very lucky."

"I'd always had this fantasy of seeing this baby grow up on a show," Franklin said. But on television, toddler actors are discarded as quickly and easily as last week's stage sets, and, anyway, not many people thought Franklin's sitcom would last long enough to have a chance of making his dream come true.

The casting was done in the usual way. They had to have a set of twins to play baby Michelle, the youngest daughter in the fictional Tanner family, in order to keep each child's working time under the legal maximum and ensure no expensive interruptions if one child became sick or difficult. In the spring of 1987, 20 infants and toddlers, the result of 10 multiple births, were brought into a room at the Lorimar studios in Culver City, a somewhat sterile television industry town south of the Santa Monica Freeway, and allowed to interact. The producers got on the floor to join in the play and began to notice one pair of 6-month-old blondes.

"There was a spark there," Franklin recalled. "They were more open to strangers. They were smiling all the time." Time was short. None of the applicants was capable of reading for the part. With little but their instincts to go on, Franklin and his colleagues selected the daughters of Dave and Jarnie Olsen of Sherman Oaks, Calif. A former golf pro turned mortgage broker and former ballet dancer turned housewife, they were about as far from the image of relentless stage parents as one could imagine.

"I didn't even want to be bothered with it," said Dave Olsen, whose wife had sent the girls' photo to a talent agency at the suggestion of a friend. "I said, 'Honey, if you want to do this, fine.' "

"Full House" debuted in September 1987 with little applause and not much better ratings. But as rerun season began the next spring, many American families like mine, flipping the dial for something new, stumbled onto the adventures of the Tanner family of San Francisco, as re-created in a huge, chilly Southern California television studio with all the charm of an airplane hanger.

The Olsen girls, at least as captured on tape, were not talking yet when I first saw them. In the first few episodes they were about 9 months old, with huge blue eyes and thin, unruly hair that the producers often pinned up to emphasize their eyes. For awhile they were little more than squirming props, with a precocious ability to move and smile on cue, but no more dramatic than trained seals.

Yet, they had something. To this day, having now met them and discovered that this mesmerizing quality is theirs and not just a product of good lighting and deft staging, I am not sure what it is they have. The small noses and often introspective looks carry an irresistible vulnerability. The capacity for quick movement and angry stares suggests creativity and passion. They still have little acting talent, nothing like the poise and polish of Raven-Symone on "The Cosby Show."

But it is difficult to stop watching them, and as the producers of "Full House" began to realize the twins' power over an audience, and to invent dialogue and bits of business for them, the program became a must, a bit of usually mindless drivel that could not be easily removed from my family's weekend routine.

There was a political dimension to this. As in many families with children, control of the channel changer in the early evening hours was tightly held by the family's youngest member. Kate Mathews, only 21 months older than the twins, saw them as spokeswomen for her needs and fears, and made life miserable for anyone who dared switch to a baseball game or the news.

Bowing to demands that I meet and write about the Olsens, I took a step into the alien world of child acting with the assistance of the twins' teacher, a veteran of the business named Adria Later.

Later became a studio teacher in 1975 after five years with the Los Angeles city schools that did not give her quite what she wanted. "I realized I wanted to move on and be somewhere in the adult world rather than be in a classroom with little children every single day, yet I wanted to work with children," she said.

Her parents knew a casting director who suggested she file an application with the Los Angeles school system, which then supervised studio teachers in the area. On her first assignment she hit one of the mother lodes of child acting -- the Little League movie saga "The Bad News Bears." That led to "The Bad News Bears Go to Japan," with the nice bonus of a trip there, plus several commercials and an occasional television show episode featuring children.

The work routine was usually the same. "Children come with school books and assignments from their regular school teachers," she said. "My responsibility was to give them their lessons three hours a day, which is required by law, and for the remainder of the day to stay with them and watch out for their well-being, their welfare."

Later worked hard, made friends easily with actors' mothers and caught the eye of child-oriented directors such as Steven Spielberg. During the nighttime filming of Spielberg's "1941," Later said, "I'd wrap the kids up in their jackets and bring them hot chocolate, which isn't on the list of requirements for a studio teacher but I guess is the mother instinct in me."

Spielberg put his arm around her to thank her and said he would have more work for her. He kept his promise, thereby qualifying Later for the studio teacher's hall of fame, if there were one. The film he brought her to work on had the working title "Boy's Life," but it became the legendary "E.T." Her son Matthew, now 19, visited the set and left incandescent, having chatted casually with Harrison Ford (whose wife, Melissa Mathison, wrote the screenplay) and watched film history in the making.

Later did not always enjoy the unpredictability and harsh conditions of moviemaking. She had become friendly with Barbara Cameron, mother of both "Full House's" Candace Cameron and ABC sitcom "Growing Pains' " Kirk Cameron. When the veteran stage parent suggested that Later help on the pilot of "Full House," she gladly accepted. "I fell in love with the children," Later said, and the Olsen girls became a special project.

With infants, Later said, "the studio teacher's responsibility is just to sit back and make sure the baby isn't put under any lights that may drop on their heads and make sure they have a clean area for resting." But from the beginning, she "went into the dressing room, got down on the floor and tried to get them used to me.

"When baby leaves Mommy, it's immediate screaming," she said. "If I'm the mediator, the in-between person, and they feel comfortable with me, then I can take the baby from Mommy to here to the actor," who in turn can deliver his lines without juggling 15 pounds of wailing, diapered rage.

The Olsens, Later said, "were very calm babies. They were smiley babies. They weren't real hyper and they weren't lethargic either. When we did the series the first year, Ashley was very much afraid to come on the set and her sister, Mary Kate, did most of the scenes." Both twins had trouble adjusting to the return to the studio after the lengthy writers' strike of 1988. Lights and sound booms that they had ignored as babies assumed monstrous proportions. Later coaxed them along.

One day Mary Kate had an eye infection and the shy Ashley was needed to perform. "I went up to her and said, 'This is your day,' and I took her away from the mother and dressed her in the classroom," the little area where she kept the child actors when they weren't taping scenes. "We took her out there and she did the whole show, and blew everybody's mind," Later said.

There have been times since when Later has found herself with an Olsen twin clinging to each leg, begging to go on. "The way it works now is that they each do about half the show," Later said. All appearances to the contrary, they are fraternal, not identical, twins, and they also "have very different personalities," Later said. "Mary Kate is now a bit more shy, and Ashley is more talkative than Mary Kate, so if it's a scene where there's not a lot of dialogue involved and just a lot of action, I may tend to use Mary Kate more. And if it's something that requires a lot of dialogue, then I use Ashley. But if there's a day when Ashley is real tired, which happens, I'll put Mary Kate in a dialogue scene and she pulls it off too."

When taping, Later usually stands or crouches off-camera at a spot that will focus the twin's eyes in the direction desired and gives the child her line, which she repeats. The twins' ability to give this parroting exercise some spark and emotion has grown with each season, and often they do not need the prompting, although the producers still do it. Television sitcoms require so many cuts and reaction shots that cutting out the prompter's words is easy. Studio audiences appear to lose none of their affection for the Olsens when they see how it is done.

The show's long run has allowed the two small girls to accept the other actors as an extended family, and the male leads -- John Stamos, Bob Saget and David Coulier -- have developed a rapport with each twin. Stamos, a bachelor, took the girls to Disneyland on a day off. Coulier, knowing that the girls as babies hated being put in their cribs, changed his line slightly in an early show to spell out the word "n-a-p" so they would not burst out crying.

Sitting with his daughters in matching cowboy outfits, watching in amazement as they performed perfectly for a photographer, Dave Olsen said he and his wife have managed to keep home life fairly normal. The twins' brother, Trent, 6, and sister, Elizabeth, 2, help create a warm, chaotic atmosphere that draws attention away from them. "They know they play Michelle," Olsen said, "but we don't even watch the show all the time on Friday night. With four kids, everyone running around, 8 p.m. comes and goes."

Ashley and Mary Kate eat Happy Meals with chicken nuggets, beg for a pony and invite over many other neighborhood children with no intrusion from bodyguards or publicists. The fame and money, being put aside for college, have stunned their parents, but "my wife and I are not overly ambitious in that area," Olsen said. "You can't believe it's happened, but it has and you just kind of go for it."

In a development certain to please the twins at least as much as their television work, the New Jersey and Hong Kong toy manufacturer Meritus Industries is about to market a 14-inch Michelle doll with the Olsen features faithfully reproduced in plastic. When hugged, the doll speaks with the twins' recorded voice, spouting Michelle-isms such as "You got it, dude" or "No way, Jose."

Olsen seemed as puzzled as anyone at what has caused this fascination. He gazed at his daughters, still mugging for a camera, and admitted he did not know their secret. "They utilize their personalities pretty well," he said. "They're just kind of funny. They were almost like chimpanzees when they were little. You just dress them up and they're fun. I guess they're just photogenic."