Growing up, I knew a pastor who had memorized the entire New Testament. It wasn't a trick. You could drop your Bible open at any page, start reading half a sentence, and he could continue on from there as long as you wished. He really did have the entire thing committed to memory.
When I asked him as a curious 12-year-old how he had done it, his answer was long on wisdom, but short on tactics. He said, "Don't overestimate what you can do in a year, but don't underestimate what you can do in a decade."
Maybe that's the difference? Actors need to memorize their lines quickly, while he had as much time as he cared to use?