Speaking to Radio Times, the actor admitted that it is a "real challenge" to meet the head writer's expectations for The Doctor.
He explained: "People ask me, 'Who do you think your Doctor is?' and I'm reluctant to think of it in those terms, because it's still a work in progress.
"Steven Moffat, who's just the most brilliant writer, told me when I first met him that the interesting thing, the defining thing, about The Doctor is that he never quite knows what's going to come out of his mouth in any given situation."
Smith continued: "His thoughts just combust spontaneously . I've tried to harness the brain-to-mouth rapidity. I mean, you could think about it forever, but how do you play the most charismatic man in the universe? It's a real challenge.
"If you play him constantly charismatic, he instantly loses the charisma. Of course, really charismatic people don't have to do a thing. They just are. I'm still finding my way on that one, but I like to think it gives me something creative to play with."