Question: I don't quite understand why they are on such friendly terms in private.
When both characters are sitting in the Study, Stephen simply sits there drinking. He doesn't ask for permission to drink or to sit down. Not only that, but he calls his master by his first name (Calvin) rather than "Sir" or "Mr Candie".
Yet, in public Stephen puts on this massive charade. And does anyone know why he pretends to have a limp? It seems so random and pointless!
I'm amazed and saddened that no one can clearly see what Stephan is due race.
He's his mentor.
It is simply the time they live in that means his true role, intellect and influence must be carefully obscured.
The other answers I think assume constant subjugation and domination in slavery and lack of superior ability and intellect from those that were owned. Slavery because its precept is irrational becomes an irrational reality, that even in imaginative viewers of fiction, coupled with racism demands Black subjugation no matter the race of the viewer. We're conditioned to only be able to perceive Stephan one way.
He is the father figure, the Aristotle, the Plato.
Much as Schultz is mentor to Django, his father figure, Aristotle, Plato.
The contrasting play is on how men were grown and groomed in this society based on the insane social construct of race. As with all fiction, its predicated on the device of what if....?
What if a White man mentored a Black man in that time period and undid, liberated him from his social shackles?
What if a slaveowner was actually mentored by a Black man?