“She wanted to deliver for the character,” Gunnar Peterson, one of the multiple trainers Heard worked with during filming, tells PEOPLE.
Peterson trained Heard, 31, four to five times a week, “for a non-stop, no-breaks hour with me, and THEN she went to her fight training, which was rigorous!” he says.
“Everything was athletically based. We trained movements, not muscles,” Peterson says. “Squat presses, sled work and lots of work in a rotational plane against resistance. She is a true athlete.”
Peterson adds that their workouts were “unrelenting,” but Heard handled it with ease thanks to her dedication.