To slim down, the "Parks and Recreation" star said he cut back on drinking. He posted a picture on his Instagram account saying "Six months no beer. #GOTG Kinda douchey to post this but my brother made me."
Pratt, 34, will star with Zoe Saldana in the Marvel film, which is set for a 2014 release.
This is not the first time he has changed his body for a role. In "Moneyball," he played a 220-pound professional baseball player. He then gained 50 more for "10 Years" and dropped some again for "Zero Dark Thirty."
“I just like to gain weight and lose weight,” Pratt said. “It’s a roller coaster. I just want to do this.”