Referring to characters in books or in real life.

[Integral]

Are you "supposed" to refer to characters by their last name or first name? I've used characters first names when writing essays and their last names when writing formal papers. Am I just supposed to use their last name?

[jgweed]

In essays or papers, the preferred usage is to employ last names, as in "Kant's theory of the transcendental aesthetic......" especially if they are well-known or the subject of the essay. Lesser-known authors should have their full name given at the beginning, and subsequently the last name should be used. In discussing a character in a novel, etc., it is best to use the name the author most frequently uses (think of Russian novels, for example).
If writing a short story, on the other hand, the literary style and subject matter of the narration more or less dictates how you refer to the characters' names. Generally, the first name is used, and either dialogue or narration is used to provide (if needed) the last name to the reader.

[kelby_lake]

I use last names for men and first names for women, unless the man is a boy referred to by his first name, when I write essays and the like.

[Niamh]

I use last names for men and first names for women, unless the man is a boy referred to by his first name, when I write essays and the like.

Same.