In the Civic Orchestra’s newly revamped agenda, maestro Daniel Barenboim plays a prominent and laudable role as the first CSO leader to take an active interest in the apprentice ensemble since Frederick Stock. Foremost among his teaching tools is having CSO guest conductors rehearse the Civic players, and to inaugurate this mentor program he couldn’t have found a better and more willing coach than his longtime pal Pierre Boulez. One of the most lucid and compelling interpreters of 20th-century music and a trailblazer himself, Boulez is sure to have some helpful pointers for the ensemble as it navigates the dense serial streams that make up Schoenberg’s Variations for Orchestra (1928). This open rehearsal also provides a rare opportunity for the public to observe this master of modernism at work. (Variations is on the CSO’s subscription program next week.) Boulez will give a postrehearsal lecture starting at 2:30 across the street in the Art Institute’s Fullerton Hall. Sunday, 11:30 AM, Orchestra Hall, 220 S. Michigan; 435-6666.