6. Dustin Hoffman - Marathon Man
In an early starring role by the very talented Dustin Hoffman, "Marathon Man" tells the story of a man named Babe who is swept up in a world of criminals and gangsters, who are all under the impression that he was involved in a double-crossing diamond scheme along with his brother Doc (Roy Scheider). Babe is also a marathon runner, a talent that saves him in many convenient instances throughout the film.
In order to play this character, Hoffman lost almost 15 pounds for the role by running at least 4 miles a day. In the scenes where his character was supposed to seem out of breath, he always ran at least half a mile before shooting. There is even a famous quote that occurred between the actors Lawrence Olivier (who plays the evil German dentist) and Dustin Hoffman, in which Olivier criticizes Hoffman's method of getting into character, stating: "Why not try acting? It's much easier."
Although it was later confirmed that this line was misquoted, it's still an indication of Hoffman's devotion to this role. In an age where these extreme levels of acting was not nearly as common, "Marathon Man" was perhaps a sign of things to come.