In reply to fxceltic: I love the books!
Regarding actor choice, I think Lee Child was involved in the decision and made a comment along the lines of....
We could have got someone who fits the correct profile who was a good actor. But we went for Tom who isn't physically the same, but will be an amazing actor.
...so basically they wanted a quality, proven actor. Also, it's clear that big names sell movies too I guess. Can't wait for the film!