One theory is a German director, transplanted to Hollywood (probably Ernst Lubitsch, but possibly Fritz Lang), was asked by a script supervisor how he would like to shoot the next scene of the day. The director responded "Mit out Sprechen!", and so this was noted as a joke on the production reports and the camera slates for the shot.
Documentary, news and reality shows have added to the meaning of MOS to mean "Man On Street" which are random interviews from the public. This may have happened when a few too many film producers stumped their interns by using the term with no requested or given explanation. So the interns, turned video producers, changed "MOS" to the opposite meaning.
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