As I mentioned, when the show began at the La Jolla Playhouse in California, she didn't have the lead, but a week before the production opened, the producers took a huge risk. They not only dropped the actress who had then been starring, but they replaced her with an actress who'd never been on Broadway before. Foster was the understudy at the time, and they felt she was so wonderful in the show after jumping in that they kept her there.
This video from 2002 is basically her introduction to the national public, a young Sutton Foster before her two Tony Awards. It's a performance of the title song done outdoors on the New York streets during the Today Show.. There's a nice interview afterwards with her (a bit breathless) and Katie Couric describing this all.