Smiling is a pretty passive expression most of the time. Given the following passage:

Did Bethany get the note? I couldn't tell without either spinning around in my chair or an Exorcist-style spinal contortion. So I sat patiently in mock-attention at the wide black board that someone was was covering in dry powder.

Compare

The bell rang and as she walked out of the room, she was smiling widely. Had she read the note? Or do people like her just find things to smile about in the ceaseless monotonies of life?

to

The bell rang and while she walked out of the room, she spoke with elated excitement. Had she read the note? Or do people like her carry a limitless bliss in their hearts betwixt life's monotonies?

I think the second one is much more active, yielding something for the narrator to realistically react to. This rule probably applies to a lot of unconscious body language in general. The kinds of writing he probably had in mind, however, are when a character smiles in response to another character by smiling. In real life, people will rarely smile to prompt another person to do something. There are exceptions to this. Perhaps the scene's tension is bound up in another character's tiniest reaction.