Who wrote the episode that had 12.15 million viewers?
CREATE TABLE "table1_24961421_1" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "u_s_viewers_millions" text
);
SELECT "written_by" FROM "table1_24961421_1" WHERE "u_s_viewers_millions"='12.15';