In writing, most facial expressions are open to broad interpretation and lead to less immersive description — by that I mean they rely on more intellectual or rationalizing interpretations than evoking specific emotions.

While it's good to use character facial expressions to make the storytelling more immersive — reminding us there are people filling the space that the pages inhabit — a sharing character’s reaction is a much more powerful technique since it can evoke the reader's memory of when they either encountered a person like that or expressed that emotion themselves.

So rather than say 'she had a tight-lipped smile' or 'she only smiled with her eyes' use 'her smile made my blood run cold' — Okay, it's a trope, but I’m not going to give away my best stuff for free — or 'her smile reminded me that winters didn’t last, that summers followed, that I shouldn’t give up hope.'

Sharing character reactions to events is a very powerful technique to communicate subtle and moving emotional elements and moments in a story.