The Bourne Identity star plays an ex-convict infected by a cancer virus in the film, which is set on a ravaged Earth in the future where only the very rich have access to medical care.
Moviemaker Blomkamp saw Damon's character Max DeCosta as a muscle-bound hunk, and hired a trainer to put the star through his paces before shooting started on the film.
Damon tells Variety, "(Blomkamp) was so specific about what he wanted me to look like. He actually had a picture with my face tacked onto this guy with this body, and they literally hired me a trainer and I went to him with the picture. It was four hours a day in the gym - and I'm not 26 anymore, but I got in shape."
It's not the first time 42-year-old Damon has transformed his body for a role - he piled on 30 pounds (13.6 kilograms) to play whistleblower Mark Whitacre in 2009's The Informant!