The 19-year-old actress had a good time filming the move as everyday was a surprise for her, reported Contactmusic.
"He's great. He's the best. He's very, very honest and self-assured; very confident and comfortable with who he is. And because of that we all felt immediately relaxed with what we're doing," Ronan said.
"Everything was improvised, really. We'd learn the dialog the night before, and Ryan would make us ad-lib the whole thing. Every day was a surprise; you didn't know what was going to happen, where your character was going to go, or what relationships were going to form," she added.
"I think his acting background did [help] in this case. He had such high respect for all of us. Anything he didn't feel was honest for our characters, he'd change. He respected the instinct of the actors because he does that himself," she said.
Ronan has starred in films like "Atonement", "Hanna" and "The Host" among others.