No secret. He was an untrained guy who started training with weights and cleaned up his diet, probably with the aid of a trainer and a personal chef. He likely added a little roids to the mix as actors tend to do for certain roles. (Tom Cruise, Nick Cage, Kevin Costner etc. have all blown up in a short amount of time and then lost it all after the movie was made.)
Off topic, a little: I find in funny that when someone sees a well built person, they always say, “What workout does he do!?” like there’s some big secret.
Ron Harris tells a great story in an article he had in a past paper version of T-mag. A newbie goes up to a pro (I think it was Shawn Ray) and asked in amazement what he does for arms. Shawn looks at the kid and says, “The same thing you do, man. The same thing you do.”