Russell Crowe lost more than 50 pounds, weighing only 176 pounds for his role (compare to 228 pounds in Master and Commander: The Far Side of the World (2003)).
After the film's release many boxing analysts and even Max Baer's son, Max Baer Jr. decried the movie for its historical liberties taken with Max Baer. Specifically, that he had killed two men in the ring (he did kill one and an opponent he'd KO'd died a few weeks after their bout for reasons unrelated to the fight) and that he took pride in that fact. Max Baer Jr. has said that his father was always haunted by the memories of killing a man in the ring.
Late in the movie as Braddock fights Baer, Braddock's children are listening to the fight on the radio, and the announcer says "nobody expected Braddock to go more than a few innings with Baer"... Innings is a baseball term, not a boxing term. He should have said "nobody expected Braddock to go more than a few rounds with Baer"...
When Braddock is viewing the footage of Max Baer killing Frankie Campell in the ring, at the end two people start to pick up Frankie's body. His left arm starts to slide off his body until he raises it slightly for a moment.
Mae Braddock: Maybe I understand, some, about having to fight. So you just remember who you are... you're the Bulldog of Bergen, and the Pride of New Jersey, you're everybody's hope, and the kids' hero, and you are the champion of my heart, James J. Braddock.
Jim Braddock: You think you're telling me something? Like, what, boxing is dangerous, something like that? You don't think working triple shifts and at night on a scaffold isn't just as likely to get a man killed? What about all those guys who died last week living in cardboard shacks to save on rent money just to feed their family, 'cause guys like you have not quite figured out a way yet to make money off of watching that guy die? But in my profession - and it is my profession - I'm a little more fortunate.