The Characterization reference article from the English Wikipedia on 24-Apr-2004
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Characterization

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A good writer must be able to assume the point of view of a child, an older person, a member of the opposite gender, someone of another race or culture, or anyone who isn't like them in personality or otherwise. Characterization is being able to assume the point of view of another person and to write from a different point of view from their own. Fiction writers must be like an actor and become another person.

A good writer makes characters sympathetic, well-rounded, or complex even though the writer may not be like the character or share his or her attitudes and beliefs. This allows for a sense of realism. For example Leo Tolstoy's novels are widely regarded as having created complex believable characters.

You have to reveal some basic facts to the reader about a character. The reader might need to know many things about a character: appearance, age, gender, educational level, vocation or occupation, financial status, marital status, hobbies, religious beliefs, ambitions, motivations, etc. Often these can be shown through the actions and language of the character, rather than by telling the reader directly.

One exception to this might be in Fan fiction, when a writer is using characters already familiar to the reader, but perhaps in a setting not shown in the series or in an Alternate universe (fan fiction) setting. In this case, the writer might include a note at the beginning to avoid reader confusion, an example being the ages of the characters.