Defending Will Smith.... Natural Will versus Fake Paul
Will Smith grew-up on the streets of Philadelphia, and in this movie role he is Pretending to be upscale rich in his accent..... just like the con-artist Paul was a street guy pretending to be upscale.
Will Smith IS Paul: faking it.
And why the director chose him. In other scenes when Will Smith reverts to his natural "street" self, like when he's flipping through the address book to learn the people's histories, then he is more natural and believable. He is playing what he lived, and it is on those scenes that he should be judged, not the con artist scenes.