Sam was still grinning, but there was a curve of lip which was far from any real humor, even that provoked by the practical jokes of a barracks bully.
Sam had stopped grinning a long time ago.
mnli hypothesis: Sam had stopped grinning a long time ago. premise: Sam was still grinning, but there was a curve of lip which was far from any real humor, even that provoked by the practical jokes of a barracks bully.