Sentence:  But now, as he paced alone in his apartment, now that he was not upon exhibition, now when there was no eye to behold him, and there was no reason to <WSD>dissimulate</WSD> or veil a single thought or feeling, his look was anything but open; the last trace of frankness disappeared; the muscles at mouth and eyes shifted; lines and planes intermingled and altered subtly; there was a moment of misty transformation – and the face of another man emerged. It was the face of a man uninstructed in mercy; it was a shrewd and planning face: alert, resourceful, elaborately perceptive, and flawlessly hard. 
Meaning of the word marked with <WSD> tags:  To practise deception by concealment or omission, or by feigning a false appearance.