From the expressions on their faces, I realized the gravity of the situation.

从他们的脸部表情我晓悟到事情的严重性。

《新英汉大辞典》

Although I could not understand everything the characters said, their body language and the expressions on their faces helped me to get the meaning.

尽管我听不懂各个角色说的所有台词，但他们的肢体语言和面部表情帮助我理解意思。

youdao

She said that even though people may try to control their expressions, most are not able to keep their feelings from showing on their faces.

她说，即是人们试图控制自己的表达方式，但是大多数人都无法阻止他们把自己感受反应到面部表情上。

youdao

Except for the unhappy expressions on their faces, they looked like they had settled in for a tailgate party.

CNN: Why a pundit wrote a novel

The expressions on their faces are still a source of horror.

NEWYORKER: Naima

Remember, it's still Saturday night and the book hasn't been out for 24 hours yet, but by noon Sunday, Katie Quinn says she's finished it, too, and both sisters now have slightly stunned expressions on their faces, as if they've just woken up from a long car trip.

NPR: Harry Potter Fans Slug Down Pages of 'Prince'