He looked at them, his mouth twitching a little on one side.

She didn't say anything for a moment, but her mouth twitched.

His mouth twitched, and he turned away for a moment.

Her mouth twitched in a smile, but she didn't say anything else.

Her mouth twitched a little, although she did not actually smile.

Her mouth twitched, almost to a smile but not quite.

The girl's mouth twitched as if she were going to laugh.

Her mouth twitched as if she was going to laugh.

I could see she was not happy with that answer by the way her mouth twitched at the sides.

Her mouth twitched a little in something that was not quite a smile.