According to Randy Roberts and John Olson’s “John Wayne: American,” MGM and producer Kenneth Hyman gave the Duke first crack as Reisman. Wayne liked the script, and was close to boarding the project. There was just one problem: the original screenplay opened with Reisman carrying on an affair with a woman whose husband was deployed elsewhere in Europe. Wayne was opposed to playing an adulterer regardless of the situation, but he especially abhorred the notion of a man knowingly sleeping with an enlisted man’s wife.
Hyman had Reisman rewritten at Wayne’s behest, but the star, who engaged in at least three extramarital affairs throughout his life, ultimately turned down the role. Marvin, who, unlike Wayne, served in World War II with valor (he’s buried at Arlington National Cemetery), seized the role and further cemented his status as one of Hollywood’s baddest motherf******.
To be fair, Wayne walked from “The Dirty Dozen” in part because he wanted to make a morale-boosting Vietnam War movie to combat the declining stateside support for the conflict. This film, “The Green Berets,” wound up being the worst of Wayne’s career. It’s brainlessly jingoistic, shamelessly xenophobic, and less convincing in its portrayal of war than a bad episode of “Combat.” Wayne stuck to his right-wing convictions, and was ridiculed for it. And thank god because, really, Marvin was the only man for this grimy job.
Source From: www.slashfilm.com