Dialog from 'The Tonight Show with Johnny Carson': His guest was Lee Marvin  Johnny said,    

'Lee, I'll bet a lot of people are  unaware that you were a Marine in the initial landing at Iwo Jima ...and that during the course of that action you earned the Navy Cross and were severely wounded.' 'Yeah, yeah... I got shot square in the bottom and they gave me the Cross for securing a hot spot about halfway up Suribachi... But, Johnny, at  Iwo  I served under the bravest man I ever knew... We both got the Cross the same day, but what he did for his Cross made mine look cheap in comparison. That dumb guy actually stood up on Red beach and directed his troops to move forward and get the hell off the beach.   Bullets flying by, with mortar rounds landing everywhere and he stood there as the main target of gunfire so that he could get his men to safety. He did this on more than one occasion because his men's safety was more important than his own life.

 That Sergeant and I have been lifelong friends. When they brought me off Suribachi we passed the Sergeant and he lit a smoke and passed it to me, lying on my belly on the litter and said, 'Where'd they get you Lee?'  'Well Bob... if you make it home before me, tell Mom to sell the outhouse!'  Johnny, I'm not lying,  Sergeant Keeshan was the bravest man I ever knew.  The Sergeant's name is Bob Keeshan. You and the world know him as Captain Kangaroo.'  

 On another note, there was this wimpy little man (who just passed away) on PBS, gentle and quiet. Mr. Rogers is another of those you would least suspect of being anything but what he now portrays to our youth. But Mr. Rogers was a  U.S.  Navy Seal, combat-proven in Vietnam  with over twenty-five confirmed kills to his name.  He wore a long-sleeved sweater on TV, to cover the many tattoos on his forearm and biceps.  He was a master in small arms and hand-to-hand combat, able to disarm or kill in a heartbeat

Well this isn't exactly true.  Lee Marvin did indeed serve in the Marine Corps, however he was wounded on Saipan eight months prior to the Iwo Jima landings.  Marvin was awarded the Purple Heart not the the Navy Cross, and discharged from the service.  Captain Kangaroo did enlist in the Marine Corps, but he enlisted too late to see combat in World War II.  The rumors are a fabrication of  faux secret society The Sons of Lee Marvin.  Lee Marvin in addition to starring in great films as The Dirty Dozen, The Big Red One, and Donovans Reef is buried at Arlinton National Cemetery.

As for Mister Rogers he never served in the military at all. 

Seriously, does this guy look like he has 25 confirmed kills!

Howver, there are other examples of real life celebrity soldiers who saw combat. 

Audie Murphy was the most decorated soldier in World War II ,and became a movie star after the war.  Murphy earned the Medal of Honor along with 32 other medals. He starred in the movie about his life To Hell and Back,  and appeared in over 40 movies until his death in a tragic plane crash.

Then there is my personal hero, Ted Williams: The Splendid Splinter, The Greatest Hitter Who Ever Lived.  In addition to being the last major leaguer to hit over .400 (.406), Williams was a fighter pilot in the Marine Corps. During World War II he served as a flight instuctor. Williams saw combat in the Korean War.  Williams nearly died when his plane took  AA fire over a target in North Korea.  He had to limp the plane back to base where it nearly exploded on landing. Williams flew 38 combat missions in Korea before being pulled from service.  Williams' military service came in the middle of his baseball career, which severely curtailed his career stats. 

Ted Williams did in real life what John Wayne only acted on screen.