The 37-year-old actress is barely recognisable playing a struggling woman in new movie Conviction.
“It’s the biggest transformation I’ve ever made on screen. It was a really intense role,” she told US chat show host Jimmy Fallon.
“I think that I just scared myself,” she added after the TV presenter showed a photo of her playing Roseanna Perry.
Lewis, who also fronts rock band The New Romantiques, said she was shocked when reporters started asking her about her teeth after seeing the movie at the Toronto Film Festival.
“I had just gotten a root canal a couple of days before and I’m in pain and I have to promote the movie,” she explained.
“The journalists who just saw the movie kept saying ‘So how’s your teeth?’ I thought ‘Oh my gosh, how did they know about my root canal?’”
Conviction, which also stars Hilary Swank and Sam Rockwell, is being shown at US cinemas now.