LOS ANGELES—Although Rosario Dawson had already survived the film industry’s idea of demise, she admits to being “scared” of taking on the role of a dying woman in Seven Pounds, which opens on December 19.

Dawson had lead roles in two of the biggest box-office failures in recent film history: 2002’s The Adventures of Pluto Nash and 2004’s Alexander. Had either of those two films—which cost a total of $255 million to make and netted less than a combined total of $40 million domestically—done at all well, she would probably be a household name by now. Although that didn’t happen, it’s quite possible that at 29, with more than a dozen years of movie-making experience, Dawson could find herself back on the A list thanks to Seven Pounds.

In an L.A. hotel room, she says that she knew the film—which stars Will Smith as a Good Samaritan offering help to several people in dire need—had attracted interest from a number of Hollywood’s leading ladies. “I remember I broke down and cried when I was with my mom, and I told her that I had been asked to audition. I said, ”˜I am so scared, because this is the most important thing ever.’ You always hope as an actress that you will get this kind of script across your desk, and sometimes I have gotten a great script and then been told that there is an offer out to someone else. I knew that if I didn’t get this part, it was because of me, that there were a lot of other women vying for this role and that this was really big. I needed to know that I could be a good enough actor to make this woman come to life, because she has an incredible story that needed to be told and needed to be told well.”

As soon as she was signed to the role, Dawson did whatever it took to get it right, including extensive research on heart transplants. She says she began to realize she was doing something completely different: rather than just playing a role, she was taking on her own fears of death and dying.

“I did a lot of research. I spent time with someone who had had a heart transplant. I studied about surgeries and I talked to a trauma doctor, and all of it brought out a lot of information about how many surgeries Emily would have had. She would have gone through four or five pacemakers because you don’t go into the level of being on a [transplant] waiting list without that, so talking to him [the heart-transplant recipient] was incredible. I learned that he couldn’t run or even walk more than three paces without being out of breath, and I filtered it through Emily, knowing they were part of her life. But it is difficult, because you start thinking about your own mortality, and that’s a lot different than battling aliens.”