She's a hired killer who's drop-dead gorgeous and should pack plenty of punch at the box office this weekend.

In "Elektra," (search) the latest big-screen superhero saga, Jennifer Garner (search) reprises her supporting role from "Daredevil," in which she co-starred with now-boyfriend Ben Affleck.

Now an active assassin, Elektra changes her tune after being sent to kill a widower and his 13-year-old daughter.

"We barely touched on Elektra's story in 'Daredevil' — and she has this fascinating saga. And in 'Daredevil,' you saw her go from a lighter, more vulnerable person to dark — and in this movie you see what happens directly after that. But the meat of the movie — she is dark," Garner told FOX News.

And while Garner's conditioning is apparent on her TV show, "Alias," getting physically prepared for this role was something else entirely.

"I train about seven or eight hours [combined] on Saturday and Sunday ... and then an hour a day in the morning and then maybe half an hour of just boxing or something at the end of the night," she told FOX News.

Indeed, the "Elektra" junket was scrapped and an elaborate Las Vegas premiere was nearly postponed after Garner was injured doing one of her own stunts.

She's on the mend, but the incident leaves little doubt she throws everything she's got into her roles.

"If I had been plunked into this without having had the last three years of experience that I've had — no way. But because I have this continual relationship with my fantastic teacher and stunt double — this is just our next thing — so yes, [the stunts are] harder, but they don't seem as daunting as they otherwise could."

With the popular "Alias" just starting its fourth season and her movie career taking off, Garner is one busy star.

"I've been lucky that people have allowed me to be different roles. I think anybody that you've stereotyped, I guarantee that they could do the opposite," she told FOX News.

And while Elektra might not be a household name, in the world of comic books, Garner has a lot to live up to.

"The name of the movie is 'Elektra' and then ... every now and then I think, oh shoot, that's me I'm that girl. I really can't screw this up."

FOX News' Bill McCuddy and Mike Waco contributed to this report.