CREATE TABLE table_24961421_1 (written_by VARCHAR2, us_viewers__millions_ VARCHAR2)

Who wrote the episode that had 12.15 million viewers?
SELECT written_by FROM table_24961421_1 WHERE us_viewers__millions_ = "12.15"
SELECT "written_by" FROM "table_24961421_1" WHERE "12.15" = "us_viewers__millions_"