Cast the Expert: A particularly cool example. Louis Ozawa-Changchien has been practicing kendo since he was 5 and did his own stunts for Hanzo's fight with the Falconer Predator, but the crew didn't know this when he was cast. He brought his kendo teacher (who he considers a second father) on to help choreograph the scene.
Adrien Brody, known for being lean and playing nice guys, plays Royce, the ripped badass loner. Word of God is that they went with a smaller guy because while real-life soldiers are in great shape, most aren't huge, burly guys like Arnold Schwarzenegger.
What Could Have Been: Laurence Fishburne's character, Noland, was originally supposed to be played by Danny Glover, which would have been the perfect way to tie the film to Predator 2 as well as 1. Though whether Glover would've been playing the same character or not is unclear.
In the dvd commentary they state that Isabella was originally planned to die saving Royce. Between her performance and the ending being Royce as "the king of death" they decided not to. Makes Royce's reaction to Isabella's injury from the Berserker more understandable.