In the 1928 movie version of Victor Hugo's The Man Who Laughs, the central character, Gwynplaine, was played by Conrad Veidt. Veidt's fixed grin and disturbing clown like appearance was a key inspiration for comic book talent, Bob Kane for creating Batman's greatest enemy, The Joker.

The above information can be found many places. From the Quotes and Trivia section at Amazon.com for those purchasing a DVD copy of the 1928 flim to a review of the movie in the 2000 issue of Monsterzine.

There is a controversy, however, that is likely never to be settled over the creation of The Joker. Bob Kane did not create the Batman comics alone, and those on his team, Jerry Robinson, and Bill Finger, have both at times tried to claim credit. The Crimson Collector has a summary of this controversy.