But, in an exclusive interview to be broadcast Sunday night on 60 Minutes, she reveals to Correspondent Mike Wallace that she came down with a dangerous staph infection while making the film.
The Early Show on Thursday aired a portion of Wallace's report, then turned to an infectious disease expert to learn more about staph infections.
"Her efforts impressed all the pros at Gleason's" gym, Wallace says.
But, he adds, Swank kept a secret from the boxers, and the filmmakers who hired her.
"You got a staph infection…that could have been fatal?" Wallace asked.
"I was about halfway through my training when I got a blister…the size of my palm on the ball of my foot. And it was my front foot, the one I have to pivot on. ...So, I popped it. …I popped it myself. I didn't let it pop on its own, whatever. I popped it on its own. And I did it in the bathtub.
Within 48 hours, Wallace says, the blister was infected. Staph bacteria were crawling through her veins.
"I couldn't believe the pain," she says. "It was -- unbelievable. And I looked down…there were streaks going up my foot. So, I went to the doctor that second. And he looked at me, and he said, 'This is really serious. And if you would have waited two more hours, you would have been in the hospital for three weeks.' And if it gets to your heart, that's it."
Swank kept it hush-hush, until she told Wallace.
"I didn't tell Clint (Eastwood, director of 'Million Dollar Baby'). He still to this day doesn't know. The producers don't know. No one knows…except my trainers…because, in the end, that's what happens to boxers."
Dr. Tim Wilkin, an infectious disease expert at Weill-Cornell Medical College and New York Presbyterian Hospital, tells The Early Show co-anchor Rene Syler staph infections are "kind of scary. A staph infection is an infection caused by a bacteria called staphylococcus aureus, or "staph," for short.
"It's actually a very common bacteria. It lives in people's noses and on their skin, and at any given time, probably one in four people has staph living on their skin. These (serous) infections are pretty uncommon. So most of the time, staph just comes and goes and doesn't cause any problems.
"Occasionally, when you get a small cut or an abrasion or a blister, as was mentioned (by Swank), staph can infect it and cause some problems."
In Swank's case, Wilkin says, "I think whether or not she popped it, the staph was infecting the blister, and it seemed to spread to the surrounding skin, and possibly, through the blood."
How do you treat staph?
"Try to avoid getting infected. Make sure that you take good care of your skin and avoid cuts and scrapes. You want to make sure that you wash your hands frequently and you don't want to share towels, washcloths, clothes and that sort of thing, because that just spreads the staph from person to person."
When do we know it's time to see a doctor? In Swank's case, she says she saw the infection was sort of creeping up her leg, Syler notes.
"It sounds like that happened very quickly for her," Wilkin responded, "and that can happen in some cases. Most often, it starts with just a small little boil or pimple or something that can be easily treated with just warm compresses, wet towels.
"The warning signs are if you notice redness spreading from that site, or if you're having any fever or chills, or any intense pain. That's something that should prompt you to seek medical attention right away."