You ask them the reason for the facial expression.
The gutsy soldier just silently grimaced.
My Grandad grimaced when he saw me kick my ball at his greenhouse.
The word grimaced is a verb. It is the past tense of grimace.
Frowned, grimaced.
scowl, frown
Example: Ashley grimaced when she saw her grade on the math test. Dictionary definition: A facial expression that indicates disapproval or pain.
frowned, glowered, glared, grimaced
I stared into his eyes gloomily. He grimaced then struck me in the face.
Another word for frown might include grimace, glower, or glare. Other synonyms for frown would include scowl or make a face.
The cab driver's face grimaced in repulsion to the scene of the accident.
Grimace is a noun and a verb. It just depends on how you use it. Noun form: The grimace of the refugees in the photograph reveals the pain of homelessness. Verb form: Most people grimace at the mere sound of the dentist's drill.
I grimaced upon reading the lack of grammar in this question. There was a grimace upon my face on reading this question.