Though Saoirse Ronan‘s character in “How I Live Now” is rough and tumble, she cleaned up for the L.A. screening of the film at the Arclight Cinemas on Nov. 5.

She said she enjoyed playing a character that was a little rough around the edges as a nice change to past roles.

“I had played quite a few characters who were very kind and very kind of sweet,” she explained. “As much as I loved playing them because I found them all very interesting in different ways, I wanted to change that a bit. I didn’t want to just kind of do that one thing.”

The young adult novel of the same name had amassed quite a following since its 2004 release, but Ronan took a different approach than many other actors might in tackling an adaptation. She admitted she waited until after they completed the film to read the book, as there were a few changes to her character that she just wanted to focus on, but she did open it up after filming wrapped.

“I think it was very emotional for me to read it afterwards,” she said. “I pictured myself and I pictured George MacKay as Eddy and Tom Holland as Isaac and the atmosphere that we have on set suddenly was playing in my head.”

Director Kevin MacDonald, however, was a fan of the book long before jumping on board. He said that as a 45-year-old man, he doesn’t spend much time reading young adults books, but Meg Rosoff’s novel was an exception.

The choice of Ronan for the lead, however, was the last thing on MacDonald’s mind.

“I wanted to have an unknown,” he said of casting. “I thought I had to have somebody who was American to do it, but I couldn’t find anybody over here that I really liked, and then I met Saoirse. She came and read a scene, and I was blown away, and thought I’d been an idiot for not thinking of her to begin with.”

He also touched on the challenges of filming, and surprisingly enough, the gun-firing action sequences weren’t the hardest parts.

“The things that are hardest to shoot are the things where you want people just to feel very natural, and you want to do love scenes and you want to do just kids hanging out and trying to get them to relax,” he explained.

Attendees headed to Wood & Vine restaurant and bar in Hollywood after the screening to mingle over cocktails. Magnolia Pictures releases “How I Live Now” in the U.S. Nov. 8.