As 'A Different World' Turns

''Ahhhh-HEMPHHH,'' says Debbie Allen, striding across a Hollywood rehearsal hall where she has taken a week off from directing and producing A Different World to choreograph the Oscar telecast (including a number showing off Guy). Barely 5-foot-2, even in a Davy Crockett cap, Allen doesn't look the part of a commander-in-chief, but she's one of the few people who can bring 28 dancers to attention by clearing her throat. ''Get in costume,'' she barks, whacking a jockstrap-clad dancer preparing to play a teddy bear in a Home Alone production number. ''let's get moving!''

''I'm aggressive,'' she admits, straddling a chair during a break. ''I can move people around and make them do what they should be doing. See? We laugh, but we work.'' Allen had won a Tony nomination for acting in the 1980 revival of West Side Story and collected Emmys for choreographing TV's Fame, where she also directed 11 episodes, when she arrived on the set of A Different World in 1988. ''I saw a show that had a lot of talent,'' she says, ''but was mindless, and being run in a dictatorial way.''

Allen had more than leadership to offer; as a 1971 Howard University graduate, she had firsthand experience at a black college. ''This show had waitresses in the school cafeteria,'' she moans, shaking her head. ''I said, 'Honey, what is this waitress shit? At this school you stand in line and you clear your own place.' '' Quickly, the writing staff was dispatched to Atlanta's predominantly black Spelman and Morehouse colleges. ''She insisted we look out into the world,'' says Fales. ''It's appalling that we were allowed to write about black colleges without having [done research]. There's a spirit of family, intimacy, and mission that we didn't know about.''

Allen made her influence felt in every corner. She had the Pit set redesigned to look more realistic, making the kitchen and its workers more visible. She oversaw new costumes to replace the rah-rah cheerleader look. She instituted a daily morning workout for the cast. ''I made them do some sit-ups and stretches. I put them through those paces because I wanted to make them an ensemble company, working together.'' Then she assembled the writers and actors.

''She said, 'Now this is what's going to happen — we're going to have a read-through, and then we're going to talk,''' remembers Hardison. ''We had spent a year doing what we were told, and now someone was listening to us.''

''Lines of communication opened,'' says Guy. ''For the first time, we saw how far this show could go. Debbie never stopped. I remember her saying, 'This is a Southern school — I want to see some grits in the Pit.'''

But Allen brought more than grits to A Different World: She brought grit. ''Debbie broke the Cosby umbilical cord by saying, 'We've got to be topical,''' says Fales. And unlike most sitcoms, which falter in ''issue'' shows, World has thrived by diving into controversy. In recent months, the series has addressed date rape, apartheid (in an episode about corporate investment in South Africa), and religion (by exploring a student's decision to become a Muslim). This season, World has traced an interracial romance, and it's hard to imagine another TV comedy including a scene like the one in which Kim was belittled by a black friend for dating a white man. This month Whoopi Goldberg plays a professor in an AIDS episode. (''I fought for a year to get that done,'' Allen recounts, ''and finally said, 'If we don't do this, one day we'll look up and The Simpsons will have done it.'")

In January World became the first sitcom to address the Persian Gulf crisis, in a hard-edged episode about a black Army reservist (Blair Underwood) that was cowritten by Guy. When the soldier asked angrily if he was fighting for oil, the studio audience applauded. The show's supervisors insisted on a retake. ''They said [the clapping] was too political,'' says Cree Summer, who plays unreconstructed flower child Freddie Brooks. ''But we were so happy, so high from that.'' So were the ratings; more than 30 million viewers tuned in to give the show one of its biggest audiences of the season. ''All I did was set those actors free,'' says Allen. ''I gave them their show.''

Under Allen's guidance, A Different World has become steeped in black culture. It's evident in set details, from the ''Support Black Colleges'' poster in the Pit to an announcement taped to Whitley's refrigerator heralding the National Council of Negro Women's 1990 Black Family Reunion Celebration. Viewers can hear it as well in references to everyone from Aretha Franklin to Zina Garrison, made without elaboration. ''We don't have to explain everything,'' Guy says. ''If viewers don't understand something we say, they'll survive without having it spelled out. So what if we're not universal? All in the Family was about blue-collar WASPs in Queens. This show is about a specific culture too.'' Rarest of all, the show numbers more blacks (and, not incidentally, women) among its producers, directors, writers, and crew than does any other prime-time series.