Bill Cosby stands magisterially onstage in New York's Kaufman-Astoria Studios, where Paul Robeson filmed The Emperor Jonesand Fred Astaire took one of his earliest screen tests. Today, Cos is a combination of the two: The emperor of TV is shooting the dance sequence that will open each episode of this season's The Cosby Show. Each year the melody is the same, the style different: Last season it was a '60s riff, the year before an 84-piece orchestral arrangement. This time, it's a rap-flavored track graced by Lester Bowie's incandescent trumpet graffiti.
Posed before a dazzlingly colorful mural depicting Harlem, Cosby mugs and does his moves as his prerecorded voice booms through the huge room: ''YO! Chill out! DON'T put your face in the MUD, pal-ee.'' Translation: Climb out of the mire of our permissive society! Get some respect for the family and for yourself! And get an education! Shorter translation: Father knows best!
But Father Time has caught up with Father Cosby, and the rest of his TV family can literally dance rings around him. Once the fastest halfback at Temple University, Cosby is now a relatively rickety 53. In his 1987 book, Time Flies, he calls himself ''a quarter-miler whose son now says, 'Dad, I just can't run the quarter with you anymore unless I bring something to read.''' On the set, a stand-in young enough to be his son tells Cosby he ought to do the hip, kinetic dance step called the Running Man ''something a little more street,'' as the kid puts it. Cosby listens, gives him a calm nod, and does a take featuring his own interpretation of the Running Man more of a Sauntering Man, exceedingly cool and very, very slow. Under control. ''Cliff (his TV character) plays against the kids,'' Cosby explains afterward. ''They're moving with all that energy, and Cliff is smoothing.''
But these days Cosby's got more kids to play against than the Huxtables. This fall, an upstart brat named Bart Simpson and his cartoon clan are taking Cosby on in head-to-spiky-head combat Thursday nights at 8. Starting in 1984, The Cosby Show helped propel NBC from the bottom to the top of the ratings. In 1990, the Fox network hopes to do the same with The Simpsons. Astoundingly, considering that Fox has 131 affiliates versus NBC's 209, Bart already has finished in the weekly Nielsen top 10 seven times since his January premiere. Bart did so well that Fox moved him from Sunday night into direct competition with the slightly declining but possibly still invincible Cliff Huxtable one of the greatest David and Goliath fights in recent TV history.