As the original production of "Roots" gave an unprecedented number of American viewers a compelling and frank lesson of human history, it also showcased the cream of black America's acting talent.

There would be more serious drama, fewer stereotypes, the critical forecasters said. Black actors and actresses waited. And the expectations became another dream deferred.

It has taken the sequel of "Roots" to live up to the original's promise of more visibility. And the first new star to emerge from the sequel, which will be aired seven nights next month, is Debbi Morgan. A 22-year-old actress with a slim resume but unmistakable talent, Morgan plays Elizabeth Harvey, the great-great-granddaughter of Kunta Kinte.

The discovery of the first "Roots" was LaVar Burton. The same success is predicted for Morgan. "The first had so much impact because it was a slavery film about a specific person, not a whole race. You knew the characters," says Morgan, the first person out on the road to push "Roots: The Next Generations." The actress, sitting in a suite at the Madison Hotel, delicately spooning whipped cream into hot chocolate, exclaimed over how the make-up process enabled her to change from an 18-year-old to a 73-year-old. "The role just kept expanding. Originally I was only scheduled to do four hours. Finally it was 12 of the 14 hours."

In the first two hours. Morgan shows a steady tenacity, nonfussy animation and, also, a spirited interpretation of the feminism and racial changes of the times. After viewing the first episode, Warner Bros. gave her a one-year development contract.

"This is all very fast," says Morgan, commenting not only on the pace but the style of publicity, the star talk.She looks 17, not 22, a spray of freckles across the full moon of a glistening face. She is sweetly noncommittal about the role's impact on her life but giddy nonetheless about the role.

"A girlfriend of mine called me one day and said the second part of 'Roots' has a lot of good roles for females, make sure your agent looks out for you. He did, and Elizabeth was the first part I read for," says Morgan, crossing her legs under her. "Believe me, everything they tell you in acting class about auditions goes right out the window when you are confronted with the producers and directors." But she survived four different readings for four sets of executives and tells that saga with the pitched squeal of someone opening a gift.

The original "Roots," based on the best-selling book of Alex Haley's genealogy, started with his ancestors, Mandingo tribesmen in The Gambia, and followed the capture of one male ancestor until the contemporary Haleys of Henning, Tenn. It cost a modest $6 million and was programmed for an unprecedented eight consecutive nights.

The sequel, which will run Sunday through Friday, Feb. 18-23, with a final episode on Sunday, Feb. 25, cost an estimated $16 million, according to executive producer David L. Wolper. It has 150 speaking parts. One of the serious criticisms leveled against the original was -- with the exception of Haley -- the lack of black writers and directors. Two black writers, Sidney Glass and Thad Mumford, and two black directors, George Sanford Brown and Lloyd Richards, worked on the sequel.

In the optimistic aftermath of the first success, predictions of improved serious television programming in general and a fairer share of nonstereotypical dramatic roles for blacks were heard. Generally that didn't happen. For black actors especially, a recession, not a bounty, has materialized. In 1979 a quality black actor on television is still an "occasion" and the plum part of the season, the only new starring role in a series, that of a joke-cracking kid, has gone to Gary Coleman in 'Diff'rent Strokes."

Yet, Debbi Morgan, part of the wave of young people who go to Los Angeles for discovery, enjoyed some of the limited "Roots" fallout. "I think the whole picture for blacks is better than two years ago. The industry is expanding beyond the stereotypes and away from buffoonery," says Morgan. She digs into the early 1970s to give examples of good roles for blacks and doesn't seem fazed by the lack of more recent evidence.

Since she made the trek from the Bronx in 1976, she has worked twice on "Good Times," and periodically on "What's Happening," two warhorses of racial comedy on the tube, starred in two plays at local theaters and a handful of commercials.

Those jobs were far from the negative images of Stepin Fetchit and Beulah but far from the weighty predictions inspired by the first "Roots." But she is young, eager, unscarred, without a hint of rage from behind her aviator glasses. Disappointments have come -- parts in Richard Pryor's "Which Way Is Up?" and the local company of "For Colored Girls" went to others -- but the "Roots" sequel is her break.

"On the set everyone was very high, very up. Since the story had been a phenomenon the first time, there was a feeling the second would be just as good or better. And the scripts were superb, anyone who couldn't really respond was not a good craftsman," says Morgan.

In the first two hours, set around the year 1882, Elizabeth Harvey is forbidden by her father to marry a light-skinned black. She vows never to love another man and leaves Henning for Memphis and a schoolteacher's career.

"In the second hour I have a dance, called 'Jump Jim Crow,' actually a protest because a whole series of incidents have lead me to believe my father is an Uncle Tom," says Morgan. She wanted the emotion and authenticity of the period, so she asked vaudeville veteran Avon Long, who plays the elder Chicken George, for some instructions. "He told me to stomp, to get the feeling, that I had to be ugly, distorted."

In the following episodes she moves back home when her sister's husband dies, earns a living picking cotton, and then takes her place with the elders on the front porch, passing down the story of Kunta Kinte

"The one thing I enjoyed was the growing up process. As I got older it would take four hours to add the double chin, the crow's feet, the eye bags. But what I liked most of all was the wisdom I felt as an older woman," says Morgan. "Then I would go back to my dressing room, take off the make-up and just be me."

Just being Debbi Morgan seems to be testing, looking with envy at the skills of the late Diana Sands, James Earl Jones and Jane Fonda, and shaping a life with an actor in a duplex on Manhattan Beach. Like most of her friends in the Bronx, the North Carolina-born Morgan wanted to marry or teach. In high school she got the acting bug, after a debut as Puck in "A Midsummer Night's Dream," and joined the workshops of the Negro Ensemble Theater and the New Federal Theater. She arrived in Los Angeles following her nine months as the lead in the road company of "What the Wineseller Buys."

"Right now I want to be a better actress. I don't think I would like to be on the other side of the camera, producing or such," says Morgan, smiling and showing off two cavernous dimples. "But after Elizabeth I would like to do some more biographics.People like Diana Sands, showing her talent and the dignity she brought to black actresses."