Duress, like fraud and mistake, may completely prevent the mutual assent necessary for the formation of a contract or sale, or it may be merely a ground for setting aside a bargain because the expression of mutual assent thereto was improperly obtained. If a man by force compels another to go through certain indications of assent, as by taking his hand and forcibly guiding it, there is no real expression of mutual assent for the act is not that of him whose hand was guided. He is a mere automaton. But in the ordinary case where duress is exercised, as generally when fraud is exercised, there is an actual expression of assent, though in view of the way in which the assent was obtained it. is inequitable to permit the enforcement of the bargain.87