Snape's Body Language

[alicepmint]

No: HPFGUIDX 71452

Snape's Body Language

During my 2nd reading of OOP I noticed during Occlumency lesson, this 
one line alarmingly stood out to me: 

"Snape eyed Harry, tracing his mouth with one long, thin finger as he 
did so." (p.531 US)

The reason it is alarming was that, (I'm most certain) it is very 
rare to see Rowling made descriptions of body language on characters. 
This is indeed the first time Snape was described with such gesture. 
She further emphasized it by repeating the "tracing mouth with 
finger" thing the second time in the next paragraph. Is it something 
of significance? of is it's just meant to be some general description 
showing Snape in deep-thinking-mode. But could there be something 
more? I hope anyone who's familiar with studies on body language 
could shed some light. What does it usually say about this person and 
what he's really thinking of the person he's staring at given the 
situation they were in. 

Alice