Reese Witherspoon faced a new challenge in portraying Susan, her character in Monsters vs. Aliens who transforms from a regular-size bride-to-be into a five-story-tall superhero: Talking like a convincing action hero.

“That’s not a voice I have in my repertoire,” she admits. “So they kept trying to coach me to be more like Arnold Schwarzenegger or Sylvester Stallone. That took me a long time.”

After years of comedies and dramas, Monsters vs. Aliens — opening Friday — is Witherspoon’s first real action movie. Luckily, it’s animated, so the most physical exertion the Oscar-winning actress had to endure was recording “effort noises,” the grunting, wincing and panting that accompany the film’s dizzying action sequences, like when a massive alien robot chases Susan through a crumbling San Francisco.

Witherspoon was particularly drawn to this project because of its strong female lead. “I’ve been offered a lot of animated films before,” she explains. “Sometimes they were playing a girlfriend, and sometimes they were playing a girl who got rescued by a guy. This was the first opportunity where the woman was at the centre or the movie.”

And the journey Susan takes in the film — after being walloped by a radioactive meteor on her wedding day — spoke to Witherspoon on a deeper level. “It’s got an incredible message about finding yourself and your identity,” she says. “Are you a person who struggles in the shadow of another person just so you don’t have to be alone or afraid or find your own strengths? Or are you someone who’s willing to take a chance and accomplish great things?”

But beyond playing a strong female protagonist, Witherspoon loved creating a female superhero, something she feels are in short supply. She remembers asking her children, Ava, 9, and Deacon, 5, to name their favourite caped crusaders. While her son could rattle off a host of names including Batman, Spider-Man and Superman, her daughter was at a loss. “She was like, ‘I don’t know. The girl in the Justice League with wings? I can’t remember her name.’”

As for Witherspoon, her favourite superhero was always Wonder Woman. “I watched Linda Carter over and over and over again,” she says. “I had my golden lasso and my whole costume that I wore for seven Halloweens in a row.”

But that doesn’t necessarily mean Witherspoon is up for portraying the famous character on the big screen. For now, she’s happy to keep her action movie roles in the animated realm. “If I never had to see the inside of a gym, I’d be a very happy person.”