Bob has a constantly guilty expression and never seems to be able to say not to anyone.

She looked up, a guilty expression on her face.

She didn't want him to see her own guilty expression.

She sipped the drink with a vaguely guilty expression in her eyes.

She looked around with a guilty expression when the door opened.

The two looked at her with the guilty expression typical of young love.

As Sergei entered, one of them turned around with a guilty expression on his face.

Reading his guilty expression, she said, "Of course you know.

He looked at her with a guilty expression on his face.

Some of it, particularly Robin's guilty expression, worried him just a bit.