I agree with James about Stewart...he definitely felt manlier in his later movies after WWII. He feels like a completely different actor when looking at young Jimmy Stewart and old Jimmy Stewart. Thank God he returned to acting...otherwise we wouldn't have had "Vertigo"
The war changed/aged a lot of them. They literally went from being boys to men.
Robert Taylor is a lot more "hardened" in his films after the war. And Sterling Hayden had been what they called a 'pin-up boy' who fell into movies by way of modeling. After the war and a considerable absence from Hollywood, he returns to the screen in much grittier roles in noir and westerns he would not have been able to do earlier in his motion picture career.
Most of them had become tougher, not only in their appearance but in their overall demeanor.