Ronan stars in the new action-thriller as a young woman who has been brought up in the wilds of the Arctic trained by her rogue-spy father to be an assassin.
"Hanna" joins the real world and goes on a mission to kill or be killed.
"When I got involved, I thought OK, it's going to be a great action movie and something really clever and really interesting, but I had no idea that emotionally it would be a very layered challenge for me," Ronan said.
"She's fascinated by everything and she's read about all these things and wanted to experience them for so long, but now that she actually is, it's just her in a constant state of awe," she added. "That's a really nice contrast because then suddenly she turns into this cold-blooded killer at the drop of a hat."
The action packed movie allowed Ronan to take a giant step away from her Oscar-nominated role in the 2007 period film "Atonement."
It also brought her back together with director Joe Wright, who pushed the teenager to the limit emotionally and physically.
"I trained for a couple of months before we started and I worked in the gym for a couple of hours a day and did martial arts for a couple of hours every day. We did that like 5 days a week," Ronan said.
"I suppose I'm the type of person that when I care about something or want to do something, I'm absolutely committed to achieve it, and that's what I tried to do on this film," she added.
"Hanna" also stars Cate Blanchett and Eric Bana. It's rated PG-13 and is in theaters Friday.