FEW who have heard William Warfield sing will ever forget his moving, bass-baritone version of ''Ol' Man River'' in the movie ''Showboat.'' It is the same lush orchestration with which he still closes most of his recitals and symphony concerts.
''I have to,'' he says, implying that his audiences insist on it.
But to his many students here at the University of Illinois, where he has been teaching voice for the last decade, Mr. Warfield is better known as teacher , counselor, friend, and cook. ''If you didn't know who he was, you wouldn't - unless someone else told you,'' says one of his students.
Over the years these students, who are inclined to call him ''Uncle Bill,'' have come to feel relaxed enough around him to dub themselves members of the ''Greater Warfield Family.''
As he tells it, the term first evolved when a group of students from New York , Chicago, St. Louis, and Atlanta decided after graduation to buy him an Oriental rug for his birthday. The ''family'' name they signed then has stuck.
''Not having a family of my own, the youngsters have just sort of adopted my house - I like to cook, and college students like to eat,'' explains Warfield, who was divorced in 1973 from opera star Leontyne Price, his stage partner in Gershwin's ''Porgy and Bess.'' Sometimes students house-sit for him while he's away from campus.
''He's very open-armed when it comes to students,'' says graduate voice student Lawrence Craig, who says he looks on Warfield as another father. ''He always wants to fix enough dinner for anyone who might happen to come by. . . . And he treats everybody like royalty.''
Warfield says his basic sweet-potato pie recipe started with his mother, who taught him to cook. He has added variations to it over the years, and the pie (along with his famous gooey butter cake) has become somewhat of an institution.
In an interview in his studio-office on campus, the handsomely tall Warfield, dressed in a khaki safari jacket and matching slacks, explained that his current teaching career is the fulfillment of a lifelong dream.
He is the son of an Arkansas sharecropper, Robert Warfield, who moved his wife and family of five boys to Rochester, N.Y., where Robert finished high school at night, took some divinity school courses, and became pastor of a Baptist church. There the Warfield sons had ample early exposure to music. William sang in the church choir and began taking piano lessons at age 9 from the church organist.
At 18 he earned a scholarship to the school of his choice by winning a national singing contest in St. Louis. He chose to study voice and piano in his hometown, at the Eastman School of Music. His eye was on teaching right up to the time he got his master's degree, he says.
But an offer to play a part in the national road show of ''Call Me Mister'' diverted him from that goal for a number of years. Bitten by the performing ''bug,'' as he puts it, he played bit parts on Broadway, made his debut as a concert artist in 1950 in New York City's Town Hall, and took part in a number of global concert and stage tours.
It was while he was in Australia on tour that he read in Hedda Hopper's internationally syndicated column that the search was still on for someone to play ''Joe'' in ''Showboat.'' He had auditioned for the part with Metro-Goldwyn-Mayer a year earlier - presumably before the search got serious - and had heard nothing. But he recorded a new tape and sent it to Hollywood. This time he heard almost immediately that the part was his.
Although individual songs and scenes on screen often represent the best of five or six ''takes'' spliced together, Warfield did the song sufficiently well on the first take to make it into the movie without editing. Excited stagehands urged Metro head Louis B. Mayer to rush over to see the taped version. Mr. Mayer was so moved by what he heard that he broke into tears and threw his arms around Warfield.
Warfield began rehearsals for a second film with Danny Kaye on Huckleberry Finn - he would have done a tap-dance routine - but it hit scheduling problems and was never completed. ''Showboat'' remains his only film.
From the start, he consciously steered away from opera, which, in his early career days, he says, was still completely closed to blacks.
''I geared my career into the things I knew were open for me . . . that had already been pioneered by (Paul) Robeson, (Marian) Anderson, and Roland Hayes, . . . so that when I came along I didn't have to fight the barrier of being accepted as a black singer of classical music. . . . And my idols were really people like Anderson, who was into concert literature.''
His admiration for the musical talent and pioneering spirit of Paul Robeson, who first sang ''Ol' Man River'' in the black-and-white movie of ''Showboat,'' remains strong. At the 1971 Chicago opening rally of Jesse Jackson's Operation PUSH (People United to Serve Humanity), Warfield sang Robeson's protest lyrics to the song: ''I gets weary and sick of tryin', but I'll keep on fightin' until I'm dyin'.''
Warfield says he likes both versions of the song - the original line ended ''. . . I'm tired of livin' and scared of dyin'.'' ''It represents to me a philosophical attitude about the very basic things going on regarding injustice and prejudice,'' he says. ''The river was the constant thing that kept on.''
Almost like an artist in residence, Warfield takes off frequently on weekends to give concerts (anything from singing Handel or Schubert to narrating Aaron Copland's ''A Lincoln Portrait,'' which recently netted him a Grammy Award), judge musical contests, or preside at various conferences. He was recently named president of the 65-year-old National Association of Negro Musicians. But it is decidedly the teaching, he says, which he now finds most satisfying as a career. ''It rejuvenates your whole approach to music.''
''He's not the kind of teacher who rails at you - he has a different way of making you work harder,'' notes Laura Poole, a senior in voice here who has had lessons from Warfield since she was a freshman.
''You always have the feeling that he's working for the good in you,'' says graduate student Lawrence Craig.
Warfield insists he is always available when students want to talk. ''To me, when a student says, 'I'd like to talk to you about something,' that's an order.''
So just in case there are hungry students around who may need to talk, Professor Warfield has already been to the supermarket today. There are collard greens and the makings of corn bread waiting in his kitchen.
''We're having an ethnic dinner tonight,'' he says.