CREATE TABLE table_29791 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code ( s ) " FLOAT, "U.S. viewers  ( millions ) " TEXT)

who are the directors of the episode that had 20.5 millions of north american viewers?
SELECT "Directed by" FROM table_29791 WHERE "U.S. viewers (millions)" = '20.5'
SELECT "Directed by" FROM "table_29791" WHERE "U.S. viewers (millions)" = '20.5'