When he turned back to face the screen his expression had suddenly become grim.

But the longer you look, the creepier the expression becomes.

She looked at him, then pulled away, her expression becoming guarded.

She did not return it; if anything, her expression became even more forbidding.

The longer he stared out the window, the more distant his expression became.

"Not worth a Continental" became a popular expression of the era.

The closer they came to Djibouti, the more serious their expressions became.

He looked at her, and his expression became suddenly mean, though he still smiled.

Unfortunately this expression has become current, but in future I will express myself correctly.

His expression became that of a man savoring some new, delightful knowledge.