Conrad Veidt, in an early version of his make-up for his character in The Man Who Laughs (1928), who later served as the inspiration for The Joker. The permanent grin was achieved by drawing back the corners of his mouth with hooks attached to the sides of his dentures. 
"Paul Leni, who made Waxworks, the cause of my coming to Hollywood, was the director of my picture, The Man Who Laughs, and we were all very happy about it. It was the Victor Hugo story of the man whose lips were cut away so that he has to go through life forever smiling, all his teeth showing in a horrible grinning grimace.  It took some months to make.  Sometimes I felt The Man Who Laughs never wanted to smile again.”
-Veidt (1934), quoted in American Gothic: Sixty Years of Horror Cinema

Conrad Veidt, in an early version of his make-up for his character in The Man Who Laughs (1928), who later served as the inspiration for The Joker. The permanent grin was achieved by drawing back the corners of his mouth with hooks attached to the sides of his dentures. 

"Paul Leni, who made Waxworks, the cause of my coming to Hollywood, was the director of my picture, The Man Who Laughs, and we were all very happy about it. It was the Victor Hugo story of the man whose lips were cut away so that he has to go through life forever smiling, all his teeth showing in a horrible grinning grimace.  It took some months to make.  Sometimes I felt The Man Who Laughs never wanted to smile again.”

-Veidt (1934), quoted in American Gothic: Sixty Years of Horror Cinema