In the movie I Am Legend there is a point where Will Smith sees that Fred the Mannequin has been moved. When he first sees him, the mannequin turns his head slightly.
It can be seen clearly in this clip.
Why (or how) did the mannequin's head move?
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This is indicative of Robert Neville's (Will Smith) mental condition - he has been without human contact and under constant stress. Under a prolonged period of such stress, he is bound to begin "seeing things."
From Neville/Smith's point of view, it would be natural to perceive movement where there was, in actuality none. In a type of 'confirmation bias' he would want to see movement, because he wants to be with people again. This is a natural explanation of anguished mental status.
A comparable scenario would be in Walking Dead Season 3 where
Rick sees his wife on the bridge(s) and even takes a phone call from her at one point, even though everybody knows she's dead.