Only on the subject of politics did he grow serious.

Her face grew serious as she looked into his eyes.

Only once for a brief space did they grow serious.

I knew that the difference between the king and my father were growing more serious with the passing of every week.

He made a face, but then his expression grew more serious.

And his own expression grew serious enough when she could see him no longer.

But the threat to her life was growing ever more serious.

But then she grew serious again, and time's mark showed in her concern.

The little man's face grew serious as they shook hands.

"I have heard what you did," he said, growing serious once more.