I think one major point is that he has created a work of literature. Douglass shows the reader some of the emotional and psychological connections between the characters and between the enslaver and the slave. He shows the reader that the situation is more complex than a stereotype would show. For example, in his descriptions of the characters, they become more than just a stereotype of a slave owner. The reader is shown several slave owners and is presented with a variety of characters who rise above stereotypical roles.
One device that makes a character more 'real' for the reader is by showing the reader contradictions in the character. For example, Sophie Auld, proves at the end to be one of Douglass's nicest master's and this contradiction of her teaching him to read and conversely treating him callously, makes her a rounded character.