Characterization is a literary term that applies to all genres of imaginative literature. In fiction, it refers to the development of well-rounded and lifelike characters that remain believable and consistent throughout a work. Often the presence of well-rounded characters is what distinguishes literary from genre fiction.
In drama, actors literally get into character; thus an undergraduate actor dresses up and wears makeup to act the role of the aging monarch in Shakespeare's King Lear. Stopping in mid-performance to take a cell phone call would be "out of character." In general, any action that is so atypical of the character as to seem radically improbable (a kind grandmother torturing a cat) is described as "out of character."
Whsomeone one says that someone is "acting out of character", they are implying thathat his person is doing somethinwouldn't normally wouldn't do. If you break down the words in the saying it will make sense. When a person "acts", they are performing or doing something. Character is the standard that a person normally follows or how a person normally acts. So when a person is "acting out of character", the person is doing something they normally wouldn't do.