Apocalypse Now 

Marlon Brando was famous for ad-libbing a tremendous number of lines in Apocalypse Now. Francis Ford Coppola, the director even estimates that only about 50% of his lines were in the script.  

This particular line comes when the character accepts the darkness within the soul, and states “The horror” as his final line. Whether people have seen the movie or not, many would recognize this line as something they have heard before and may recognize it from the original book that the movie is based on, Joseph Conrad’s “Heart of Darkness.”