"We went on a gun range to do some training and she was the best shot - by a long way. So yes, she knows how to take care of herself," said the former Spooks star.
"I'm a boy, you know. I would do that stuff for free, let alone to do it opposite Tom Cruise," he joked.
"That's the fun stuff. It required a fair bit of training - gun training, how to drive those cars at speed on wet roads and try not to kill anyone, which nearly happened a few times."
The 36-year-old actor added: "There were big action-set pieces and cinematic moments, but the reason why you became an actor is not to just be driving fast cars, although that is a lot of fun. It's to portray human beings in situations where they are talking to each other. You have both things in this movie."
Jack Reacher opens in cinemas on December 26.