(idiomatic, drama) Not in character; not successfully performing within the mindset of a given character in a theatrical performance. See also break character, drop character.
(role-playing game) Not acting; not "on"; behaving within one's natural personality rather than that of a character, or taking actions entirely outside the fictional context.
Adverb
(idiomatic) In a manner inconsistent with one's usual and expected personality or behavior.
(role-playing game) Away from the mindset, personality, or behavior assumed for a role that an actor is rehearsing or performing, or that a player is playing.