不动声色

Set one's face like a flint.

Keep a straight face.

keep one's countenance.

Bit one's lips.

Keep quiet.

As quiet as a mouse.

eg：He sat there, setting his face like a flint and tried to find out who was the suspect.

同义词：面不改色 Without turning a hair.

反义词：虚张声势 Barking dogs seldom bite.