CREATE TABLE table_16788 ("Series #" FLOAT, "Episode title" TEXT, "Writer ( s ) " TEXT, "Director" TEXT, "U.S. viewers  ( millions ) " TEXT, "Original air date" TEXT)

Who was the director when there were 13.66 million u.s viewers?
SELECT "Director" FROM table_16788 WHERE "U.S. viewers (millions)" = '13.66'
SELECT "Director" FROM "table_16788" WHERE "U.S. viewers (millions)" = '13.66'