Sentence:  Captain Edward Carlisle, soldier as he was, martinet as he was, felt a curious sensation of helplessness seize upon him as he met her steady gaze, her alluring <WSD>smile</WSD>; he could not tell what this prisoner might do. 
Meaning of the word marked with <WSD> tags:  A facial expression comprised by flexing the muscles of both ends of one's mouth, often showing the front teeth, without vocalisation, and in humans is a common involuntary or voluntary expression of happiness, pleasure, amusement or anxiety.