Warren Casey, 53, co-author of the hit musical “Grease.” Casey was best known as a collaborator with Jim Jacobs of Chicago on “Grease,” a joyous look at high school in the 1950s that went to Broadway and then became a movie. Among its stars was actor John Travolta. A native of Yonkers, N.Y., Casey taught in New York high schools and went to Chicago in the 1960s. He met Jacobs while acting with the Chicago Stage Guild. In Chicago on Tuesday of AIDS.