He was played by Sam Elliot in the movie with Mel Gibson.
From Daily Mail article: Joe Galloway, author of the book along with Hal Moore, said several of Elliott’s gruff one-liners in the movie were things Plumley actually said, such as the scene in which a soldier tells the sergeant major good morning and is told: ‘Who made you the (expletive) weather man?’
Sgt MacKenzie - We Were Soldiers Soundtrack with Lyrics
http://www.youtube.com/watch?v=BB2Ad04mukI&feature=related
May you rest in peace Command Sergeant Major, may angels speed you to your home.