Keanu Reeves: Chris, Platoon
Keanu Reeves was up for the role of Chris in Platoon, but as director Oliver Stone revealed in a 2011 interview with Entertainment Weekly, "Keanu turned it down because of the violence. He didn’t want to do violence."
The role went to Charlie Sheen, who had auditioned for the film in 1984 as "a dumb-struck 17-year-old." When he came back in to read again in 1986, "he’d grown and seemed perfectly wide-eyed and had a vaguely privileged look."