The adjectives impassive and passive may seem like they should be opposites (im- sometimes being a negative prefix), but they are actually somewhat similar in meaning, especially when they describe people. Someone who is impassive lacks emotion or doesn’t show emotion. Someone who is passive is inactive, submissive, or nonresponsive. Impassiveness is a lack of emotion, whereas passivity is a lack of responsive action.
More generally, passive means (1) receiving an action without acting in return, (2) accepting without resistance, and (3) existing active or concerted effort. Impassive is mainly confined to the sense noted above.
Examples
Impassive
Calm, almost impassive, she removed two latex swim caps and shook excess water from her green Speedo, then swaddled herself in a towel. [Sacramento Bee]
Politicians—once such an impassive breed—now make shameless use of ‘the welling-up moment’. [Daily Mail]
Television viewers saw the close-up of Cutler, a man standing alone, looking uninjured and impassive. [Los Angeles Times]
Passive
The law reserves assisted-suicide charges for cases in which a person takes a passive role in someone’s suicide, prosecutors say. [New York Times]
Schools and communities should encourage passive observers to become proactive. [NorthJersey.com]
Until Tunisia’s Jasmine Revolution and Egypt’s Nile Revolution, both were considered passive and docile societies. [Mail & Guardian Online]