Filming was not off to a great start on the movie set at Hollywood’s Warner Bros. Studio in May 1942. The script was only half-completed, the leading lady didn’t know how to play her part, and the male star was hesitant because he had never been in a romantic role.

So how did “Casablanca” become the iconic film it is today, reportedly the most watched movie in the world and almost certainly the most quoted — “Here’s looking at you, kid,” “Round up the usual suspects,” “We’ll always have Paris” — in history?