His abs received an executive producer credit.
About three-quarters of the way through the film, Pitt's Achilles faces off against the noble Hector, played by Eric Bana, and kills him while his entire family watches, dragging Hector's corpse around the walls of the city just in case any of them missed it (Achilles was a bit of a dick). However, while filming the scene, the fight did not go nearly as well for Pitt: While performing a difficult jumping strike against Bana directly into his raised shield, Pitt landed awkwardly and injured his leg.
Pitt's most awkward scene not involving his accent.
Or, more specifically, he fucking tore his Achilles tendon.
The only area of Achilles' body that was vulnerable, according to myth, and consequently the only part of the human anatomy that bears his name, was the one area on Brad Pitt's entire body to sustain injury while playing Achilles in a movie. Yes, it turns out the Greek gods are real, and they mock us.
It wasn't a particularly light injury, either. The filming of some scenes in the movie had to be delayed for a couple of months because Pitt could only sort of hobble around, which greatly inhibited his ability to run around trolling the people of Troy. When filming did restart, they celebrated Pitt's recovery by shooting the scene where Legolas lances an arrow through Achilles' eponymous heel, in the precise location of Pitt's injury.