Joan Cusack will replace Allison Janney in Showtime's new dramedy "Shameless," starring William H. Macy as widower whose drinking and carousing leaves his eldest daughter (Emmy Rossum) to care for her five younger siblings. Cusack will play "an agoraphobic Chicago housewife with a sexually active teenage daughter who strikes up a special friendship" with Macy's character. Janney , who appeared in the original "Shameless" pilot, is tied up on "Mr. Sunshine," which premieres at midseason on ABC. "Shameless" starts shooting this month for a Jan. 9 debut on Showtime.