"I was on tour with the band this summer. They invited me actually on Twitter," Rossum said Thursday on the radio show Valentine in the Morning. "That's how we met – on a dare. They dared me to come sing with them a song that I'd never sung before ... So I ended up on tour with the band, and it was very, very fun."
Their relationship came to light this fall after they'd been spotted together multiple times. Rossum, 23, split from husband Justin Siegel in August – though her marriage was not widely known about.
Asked why she kept the fact that she was married a secret from the public, the actress says, "It's hard to have a private life and keep it private and hope that people don't pick you apart."
As for the divorce, Rossum says, "I'm not asking for anything except freedom so that I can move on to another chapter in my life, and find happiness."