Cooper, 40, who is nominated for Best Actor in Oscars for his role, said packing on 40 pounds of muscle was not an easy job for him.
"I had to get to the point where I believed I was him. At 185 pounds, it would've been a joke... Chris wasn't ripped. He wasn't sinewy. He was just a bear.
"It was a real shock to my body. If it's pizza and cake, that's one thing. Putting 6,000 calories a day in your body gets old quick," Cooper said.
"Without that kind of caloric intake and the ability to recover, he probably wouldn't have made it," Cooper's trainer Walsh said.
"You could see him becoming Chris Kyle. His beard grew, and he just got burly. He looked forward to coming in every day and seeing what he could do," Walsh added.