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Langsam's Law originated with Devra Langsam.
"In the early zine days, when Langsam’s Law was the rule of the day: 'Don't Make Him Say That.' As explained by Paula Smith (in an essay in Warped Space #50, January 1984), this is a 'special caveat for writing media-based fiction. Don't make an established character do or say something out of line with his established character, of if you must, give good, solid reasons why'." Fanfic Symposium
There is an early mention of this term in S and H #9 (April 1980).