A Stock Phrase used on characters frivolous -- or apparently frivolous -- in the face of a Matter of Life and Death. "This isn't a game." or "We are not playing a game." (Or sometimes "We are not playing.") Sometimes even when it is a game, but the stakes are too high to treat it as one, such as the second type of Serious Business; alternatively, the character may point out about the game "This is serious," generally with an enumeration of the consequences. (With the first type of Serious Business, use of this is a characterization trope: the character is really that out of touch with reality.)