If I read the descriptions of Jack properly in the books, he's a very unassuming man. Not particularly tall or large and doesn't really stand out in the crowd. He's not a bad looking guy, but not super handsome either. With that being said, I picture someone like Misha Collins possibly playing Jack. If you've seen him on the show Supernatural as Castiel, he can play warm charming and funny, but also flip a switch and play dark really well also.
Has to be Tom Cruise. If he can play Reacher, he can play anyone. Lee Child likes to take digs at people who do things he does not like. He had an ineffectual character named Baldacci in one recent book as his (Baldaccis) character of John Puller is a mediocre rip off and his most recent continually describes Reacher in terms that make Paul Bunyun seem puny. And while I am on the subject compare and contrast Deep Sleep by Gregg Iles with Sibs by fpw.
Strangely, stumbling across this iteration of this thread after many years away, I also immediately thought of Chris Pratt.
At first Ford had formed a theory to account for this strange behavior. If human beings don't keep exercising their lips, he thought, their mouths probably seize up.
After a few months' consideration and observation he abandoned this theory in favor of a new one. If they don't keep on exercising their lips, he thought, their brains start working.
I like Chris Pratt in anything, lately. Star Lord, though tall, would make a good RJ. It would be better than the midget playing Reacher.