Tongueless belm
That's a beautiful combination of words, and is also spot on.
If I was qualifying the expression I'd say it was "tight" or "strained". I haven't seen the scene that still is from so I don't have that context, but rather than haughty or supercillious, I'm seeing it more as a sort of exasperated charade of being pleased/content, thinly veiling worry or unhappiness.
Not a million miles away from the taut purse-lipped smile Renee Zellweger pulls a lot in the
Bridget Jones films (generally when dealing with humiliating situations or fending off invasive questions about her love life - again, can't remember exactly what's happening in the below scene, but you get the idea)
