The Oscar-nominated actress plays Melanie Stryder, as well as an alien called Wanderer, in the upcoming adaptation of Stephenie Meyer's novel of the same name. Ronan revealed that she was surprised by the physical requirements for the role.
"It actually is very physical," Ronan told MTV News. "I didn't expect it to be so physical. I've been on a harness quite a bit."
She went on to say that she has particularly enjoyed performing the stunts the film requires, including jumping considerable distances with the aid of a harness.
"It was 30 feet in the air, so that was pretty cool," Ronan recalled of one stunt.
The Host tells the story of an alien invasion in which aliens called 'Souls' invade the bodies of humans and wipe out their victims' consciousness.
The movie opens in cinemas on March 29, 2013.
Watch a teaser trailer for The Host below: