smiling, relaxed, smoking, sometimes? listening to swing...yet when you see them portrayed today in movies or video games they appear as looking very, very serious
Because the footage shot back then was in areas away from the front lines. And they wanted the folks back home to see their sons and brothers were ok. You gotta remember they didn't want people in the states to know how truly horrid combat was and the strain it put their loved ones through. When they made the documentary "With the Marines at Tarawa." the president had to approve it for public release because it had raw film footage of combat and showed the bodies of Marines on the beach, where they fell. It was a shocking film for the day and FDR approved it for release because he felt the American people needed to see what was happening, that they needed to understand how important it was that we win the war.
Propaganda for 1, people back home were shown these videos and get the impression that our soldiers were doing fine and all that.
2nd is in movies character personalities are often extreme and over exaggerated. The protagonist is very heroic or an anti-hero and the comic relief character is waay to obnoxious etc. So its extreme to emphasize their roles and the mood of the movie.