The Deadpan reference article from the English Wikipedia on 24-Jul-2004
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Deadpan

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Deadpan is a form of comedy in which something humorous is said or done by a person, while not exhibiting a change in emotion or facial expression.

Buster Keaton, known as the "Great Stone Face", became famous for never cracking in a smile in any of his films.

Daria Morgendorffer, the star of the animated TV show Daria, is known for her monotone voice and expressionless face.