out of character

Short definition from Wiktionary

Prepositional phrase

1. 1
   Inconsistent with someone's personality, disposition, or usual expected behaviour.
2. 2
   Not in character; not in the zone; not successfully performing within the mindset of a given character in a theatrical performance.
3. 3
   Not acting; not "on"; behaving within one's natural personality rather than that of a character, or taking actions entirely outside the fictional context.

Adjective

1. 1
   Alternative form of out of character.