She later returned to New York, where she was the last to audition, in full costume and make-up, for the role of Christine Daaé in the onscreen adaptation of composer Andrew Lloyd Webber's The Phantom of the Opera.After having nearly missed the audition due to a family engagement, In 2006, Rossum appeared in Poseidon, Wolfgang Petersen's remake of the disaster film The Poseidon Adventure.
Over the course of five years, she sang onstage with the chorus and had the chance to perform with great singers such as Plácido Domingo and Luciano Pavarotti.
She starred in the science fiction film The Day After Tomorrow (2004) and received critical acclaim for her performance in the leading role of Christine Daaé in The Phantom of the Opera (2004).
She has since starred in Poseidon (2006), Dragonball Evolution (2009), Dare (2009), Beautiful Creatures (2013), Before I Disappear (2014), You're Not You (2014) and Comet (2014).
Rossum joked in interviews that her vocal talent and affinity for music developed because her mother always listened to classical music and operas while she was pregnant with her.
By 12, Rossum had grown too big for children's costumes.By June 2013, Rossum was cast as the female lead opposite Justin Long in the independent romantic comedy Comet, which was directed by Sam Esmail, to whom she later became engaged.