The reason why Jack Nicholson got to play McMurphy was because Kirk was too old and Michael was too young?
What a role... among the best acting in film history.
IIFC, Kirk bought the rights and couldn't get anyone to invest in getting it made. After a decade he was too old. His son Mike thought he could get it produced but wasn't a movie star (he was doing TV then). They needed a bigger name to get anyone behind it.
shareMurphy's age could be moved up or down a bit, either Douglas could have played it, although Kirk would have had to play it rather earlier. By the early 70s, being over 30 was unfashionable.
What I hope is that they both realized they weren't up to the role. I mean, both Douglases can be good in their range, but shee-yit - Kirk would have eaten enough scenery to destroy the entire mental hospital and walk out of the ruins, and Michael would have been just hopeless. Nicholson got the job done.