The actor, who stunned fans in 2009 by unveiling an 80 pounds (36.2 kilograms) weight-loss, was against bulking up to play the moviemaker in Hitchcock, which examines the story behind classic horror movie Psycho.
He instead donned prosthetics and padding to portray the portly director, and is delighted with the results.
Hopkins is quoted by Britain's Daily Express as saying, "I wouldn't put on weight. At my age it might never come off again!
"Instead, the make-up guys did some incredible work with prosthetics and padding. It takes a while to put on but it's a hell of a lot easier to get off."