out of character

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective idiomatic Inconsistent with one's personality, disposition, or usual expected behaviour.
• adjective 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.
• adjective idiomatic, drama, role-playing games 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.
• adverb idiomatic, drama, role-playing games Away from the mindset, personality, or behavior assumed for a role that an actor is rehearsing or performing, or that a player is playing.