An unlikely star was born at some of New York City Mayor Michael Bloomberg's post-hurricane press conferences: sign language interpreter Lydia Callis. Everyone from NPR to New York magazine to Gawker's own Max Read has been singing Callis' praises since she stepped up and, with her boisterous American Sign Language routine, served as a spot of sunshine in an otherwise horribly dim time.

But though America's hearing masses seem to love Callis' exaggerated expressions, she's not doing it for them. All that animation is actually very necessary to perform her job properly, according to a linguistics paper out of CUNY-Queens College last year [PDF]:

Mouth shape, eye-brow height, and other face/head movements are a required part of ASL, and identical hand movements may have different meanings depending on the face/head. Facial expressions change the meaning of adjectives (e.g., color intensity or distance magnitude) or convey adverbial information (e.g., carelessly or with relaxed enjoyment). The head/face indicates important grammar information about phrases … A sequence of signs may have different meanings, depending on the head/face;e.g., the ASL sentence "JOHN LOVE MARY" without facial expression means: "John loves Mary." With a yes/no facialexpression, it indicates "Does John love Mary?" With a negative expression and headshake added during "LOVE MARY," then the same sequence of signs indicates "John doesn't love Mary." (Facial expressions are timed to co-occur with hand movementsfor signs during specific parts of a sentence.) Further, ASL signers also use facial expressions to convey emotional subtext. Thus, facial expressions are essential to the meaning of ASL sentences.

Now you know.