Will Smith
Smith snagged the role of Muhammad Ali when he was pretty young in his career. Smith was 6'2 and 185 pounds at the time, which wasn't really an accurate depiction of what Muhammad Ali looked like at all. So, Smith worked on 35 pounds of boxer's muscle to his frame. Additionally, he had to change his entire lifestyle to make his appearance convincing...
Float Like a Butterfly, Sting Like a Bee
Smith did a ton of running and weightlifting while he was training for the role. In the beginning, he was only able to bench 175 pounds, but in the end, Smith was able to bench 365 pounds in two reps. He was even given an Academy Award for Best Actor for Ali.