Experienced interrogators can detect if someone is lying from even the most subtle expressions of body language; in fact, many of our gestures reveal our true thoughts and intentions, despite our efforts to suppress them.
I was once caught out at a job interview because I coughed unprofessionally. Who would have thought that clearing my throat would have communicated idleness, unwillingness to co-operate and a tendency towards corporate theft. What’s worse, on the way home the taxi driver refused to let me in his car because I hailed him down miserly. How he knew I wouldn’t tip him is beyond me.
Bella does so much whispering and muttering it’s no surprise that many of her co-characters have to interpret what she wants to say. She might shrug in order to feign nonchalance, but only the keen instincts of friends and enemies understand her true motives. Shrugging guiltily will get you busted every time.