I think Will playing against type would've been more interesting...and there were just moments early on where Jamie didn't exactly connect. Jamie's a strong actor, but Will's natural charisma would've just gotten something more across. It didn't exactly make sense why Django was so special until the second half of the movie. Will Smith sweats special-ness. Once you see him, you know it's an event.
And you could see what Jamie was doing, with the softness of his voice. He was trying to show how internalized the sweet slave was in the face of white people. You don't get no bass in your voice around massa, and I understood that. But with Will...outside of Morgan Freeman, he is the most non-threatening black actor. period. That sweet face is the only face he shows to the public, so suddenly it's the other side. The side that Jame does with ease that's something special to see from Will. You'd literally be watching his public persona evolve into a thinking, self-criticizing assessment of what it means, and maybe why he had to be so non-threatening to get by.
That persona, and that affect...in this role. I mean that's Travolta dancing disco in Pulp Fiction. The things the film had to say about the special negro with a talent that gets a pass, mean more with Will as the lead. It uses the last 20 years of Will being America's favorite black man to say something about what that means, and the divide between giving a special black person a pass, and being decent to black people.
And I don't doubt the performance Quentin could've gotten out of Will, wouldn't be something special.