The essence of Reacher I guess. Someone that you never know how they're going to behave, unless you get to know them well, which very few people do. More of a Vinnie Jones than the actor on TV.
That said, I'm enjoying the TV, it's doing the best it can, given that the book is written in the 1st person, so, on screen, too much time has to be spent on exposition which, on the page, is internal.