Stunt training would explain at least part of it. He probably trained in some martial art to learn how to safely act out a fight and to keep himself in good enough shape to do it.
This feels like a Pulp Fiction reference for some reason. Especially considering the conversation the two had last chapter. Very Sam L. Jackson and John Travolta.
Some actors train to make their parts more believable...performance wise. So it's likely he trained in martial arts for his roles...or had training before them like Bruce Lee, Jet Li, Jackie Chan, etc.
Good actors in action movies know actual martial arts and know how to use a gun. It's not just for show. If you don't have the relevant skills you are not going to be able to act it out on screen in a convincing manner. Look at the behind the scenes for movies like Heat. It takes months of training with real military instructors to pull off the fire fights in the film.