sergeant

It's a well known fact of life (and cinema) that a drill sergeant, second only to British Palace Guards, are the hardest people on earth to get to react to literally anything. Let alone smile.

People may try, and often, they will fail. But every once in awhile, they are able to coax a little wry grin from the drill sergeant. It may be because they did something dumb, or unexpected, but hey, at least it happened.