During the duration of filming, Josh Brolin could only shave off half his beard to accommodate the prosthetic on the other side of his face. This meant he spent several months walking around with only half a beard on his face.
Josh Brolin later admitted that the film became unsalvageable on account of its troubled production, but insisted that everyone involved, cast and crew, had sincerely wanted to make a good movie.
Josh Brolin initially hated the script, but later changed his mind, growing to like its tongue-in-cheek tone.
Jonah Hex's ability to bring the dead back to life and speak to them is confined to the film only. He does not possess this ability in the comic books.
In an anecdote on The Tonight Show with Conan O'Brien (2009), Josh Brolin revealed that he broke and repaired John Malkovich's thumb while shooting a fight scene.
Brent Hinds: (at around 11 mins) Vocalist and guitarist for progressive metal band Mastodon makes a cameo appearance during the scene where the train carrying U.S. soldiers comes under attack. Hinds is wearing a U.S. Army uniform and can be seen among the other soldiers. Mastodon, recruited by the film's director Jimmy Hayward, wrote the soundtrack for Jonah Hex.