Heath Ledger didn't work for a year because he only got offers for teen heartthrob roles. He was about to quit acting, and return to Australia, when he was cast in the film.
When teaching Mel Gibson and Heath Ledger how to shoot a muzzle-loading rifle, technical advisor Mark Baker gave them the advice to "aim small, miss small," meaning that if you aim at a man and miss, you miss the man, while if you aim at a button (for instance) and miss, you still hit the man. Gibson liked this bit advice so much, he incorporated it into the movie, just prior to the ambush scene.
The historical accuracy of the costumes and settings was overseen by the Smithsonian Institution. It's the first time the Institution ever worked directly on the production of a movie.
Heath Ledger said researching the American Revolutionary War for the film answered his question of why Americans "wave their flag so high." "It's because they went to hell and back to build their country."
Actual amputees were cast to play soldiers who lost their limbs.