They are not required to be true.

A man with an imagination and culture may feign either of them without knowing the ways of men.

To be realistic you must know accurately that which you describe.

How often do we find in novels that the author makes an attempt at realism and falls into a bathos of absurdity, because he cannot use appropriate language?
"No human being ever spoke like that," we say to ourselves,--while we should not question the naturalness of the production, either in the grand or the ridiculous.
And yet in very truth the realistic must not be true,--but just so far removed from truth as to suit the erroneous idea of truth which the reader may be supposed to entertain.

For were a novelist to narrate a conversation between two persons of fair but not high education, and to use the ill-arranged words and fragments of speech which are really common in such conversations, he would seem to have sunk to the ludicrous, and to be attributing to the interlocutors a mode of language much beneath them.