Actress Elle Fanning spent days "stressing out" after discovering she would have to memorise a lengthy three-page sermon for new movie Live By Night.

The teenage star's character, Loretta Figgis, becomes a preacher in Ben Affleck's upcoming period crime drama, and Elle admits she signed on for the role because she knew it would be a big challenge.

"After I read the script, Loretta, she's sprinkled throughout the movie, but it's a very intense part," she shared on breakfast show Today. "I knew it would be really challenging, I was drawn to that."

But Elle, 18, wasn't quite prepared for the anxiety she would suffer as she tried to perfect a key sermon scene.

"I was nervous because I normally just learn my lines for scenes pretty much the night before so it's fresh, but for this one the sermon was written out and it was three pages long in the script, and I was like, immediately when I got the part, I was like, 'I've got to start learning (it),'" she said. "I was like, 'I can't do this', and I was stressing out, so I had to learn it like a couple weeks in advance."

The teen ended up conquering her fears for the shoot, and she imagines the experience is similar to performing on stage.

"It felt like theater when up there (on the podium)...," she explained. "I'd never done a play before but when we had that sermon scene, it felt very theatrical, like a monologue or something, which I'd never done."

Ben wrote, directed and starred in the movie, which also features Sienna Miller and Zoe Saldana.