From Frankenstein (James Whale, 1931) -- the great Dwight Frye, as Dr Frankenstein's crippled assistant Fritz, stops to adjust his sock on the way up to the laboratory where the monster lies waiting to be born.
The first time I noticed this seemingly unmotivated and random bit of business, I was in a college dorm, watching the film on late night TV and (quite frankly) under the influence of a mind-altering drug. Needless to say, I practically fell off the sofa in amazement at the sheer brilliance of it. Goddamn, James Whale was some kind of genius of the prosaic!
Since then, of course, I have come to the conclusion that Frye probably just needed to pull his sock up. Although I watched the scene again on YouTube before posting this, and damned if the moving camera doesn't seem to stop moving when the actor does. As if the move was, you know, planned in advance.