Usage patterns
〈人〉は無表情だsomeone's face is expressionless
He's always expressionless, so I can't tell what he's thinking.
無表情な顔a blank, expressionless face
She stared at me with a completely blank face.
無表情を装うto feign an expressionless face
I was surprised, but I kept a straight face and continued talking.
無表情で〈動詞〉to do something with a blank expression
He answered the question with no expression on his face.