The wide mouth softened into the beginning of a smile.

Her lips softened into a smile as she looked up at him.

She went on staring at him, but her mouth softened into the beginning of a smile.

He gave me a concerned look that softened into a shrug.

Her mouth softened into a smile that seemed to come easily and naturally.

The stern expression on his face softened into a smile.

I could see the charm working on him as he softened and fell into a complete state of trust.

Sam's face softened into a private smile, as though he were recalling a special sexual experience.

For a moment the grave, tired lines of her face softened into a smile.

His hard, dry statement needs some little editing to soften it into the terms of real life.