Playing a super villain is hard work!
“She was a total trooper,” Hecker tells PEOPLE. “It was very satisfying to see her body change into exactly what she wanted it to look like in such a short time.”
During their sessions, Hecker and Robbie, 26, sculpt and tone though Pilates, ballet and other fitness modalities.
“We also mix in non-bulking cardio such as jump rope, rebounder and ballet jumps – which are surprisingly the most challenging form of cardio I’ve ever come across,” she says.
RELATED VIDEO: Chiwetel Ejiofor Dishes on Locking Lips with Margot RobbieRobbie normally meets with Hecker four to five times a week for hour-long sessions, but she kicked up her training because her body was really showcased in the film.
“There’s a scene in Suicide Squad where she changes clothes in front of a bunch of dudes, so her abs needed to be impeccable,” says Hecker. “And in those tiny shorts Harley wears, her butt was constantly on display.”
To tone up Robbie’s booty, Hecker used ballet-inspired moves with added weights.
“We did a lot of heavy-weighted, low reps of arabesque pulls hooked up to resistance pulleys, as well as arabesque lifts with heavy ankle weights in order to build and lift the butt,” she says. “We also did a ton of side series outer thigh work, targeting outer glutes with high reps and low weights, to pull those muscles in and create a beautiful line from the waist to the upper thigh.”
Suicide Squad is in theaters now.