While filming their knife fight scene, Thomas Jane accidentally stabbed Kevin Nash. Nash accepted cold beers from the film crew as compensation. Years later Jane described the incident saying, "What made it worse was that (Nash) was so nice about it. He just sat there, looked at the knife, looked at me, and shook his head."
Thomas Jane trained for nearly 7 months with the United States Navy SEALs and gained more than 20 pounds of muscle in doing so.
(at around 1h 21 mins) Rebecca Romijn revealed that in a scene where she sews up a knife wound on Thomas Jane, she pushes the needle too far in and ends up actually sewing a couple of stitches on Jane's body instead of just the prosthetic wound.
Thomas Jane initially turned down the role twice, as well as a part in X-Men (2000), as he didn't see himself as a superhero actor. He only became interested in the character after being asked to play him for the second time and he had seen Timothy Bradstreet's artwork of the character.
Jonathan Hensleigh actually knew someone who parked illegally for two years in Manhattan by using the same bogus fire hydrant ploy Frank Castle uses to keep Livia's parking space open during his schemes.