Here's the killer workout Margot Robbie did to get in shape for 'Suicide Squad'

Margot Robbie always looks amazing, but to wear Harley Quinn's short shorts in "Suicide Squad," the actress only had three weeks to get in killer shape.

The 26-year-old Australian actress's fitness trainer Andie Hecker told People that Robbie worked out three hours a day for three weeks during filming in order to get toned for her role as a maniacal supervillain in the DC Comics film.

"She was a total trooper. It was very satisfying to see her body change into exactly what she wanted it to look like in such a short time," Hecker told People.

When she isn't filming, Robbie generally meets with Hecker four to five times a week for hour-long sessions. But because she'd be showing off a lot of her body in "Suicide Squad," the actress decided to up her training sessions and go into fitness overdrive.

"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," Hecker said. "And in those tiny shorts Harley wears, her butt was constantly on display."

Hecker helped Robbie sculpt and tone her body in such a short amount of time through virogous sessions of pilates, ballet, and other fitness modalities.

The amped-up workout routine also mixed in non-bulking cardio, including jump robe, rebounder (a type of trampoline), and ballet jumps, which Hecker said are "surprisingly the most challenging form of cardio" she's ever come across. And, given Harley Quinn's barely there ensemble, the two placed a huge emphasis on working out Robbie's backside.

"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," Hecker said.

"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.