20 Things You Didn't Know About It's A Wonderful Life
9. So Many Real Tears
As aforementioned, James Stewart acted for It’s A Wonderful Life so soon after battling in World War II, and was still suffering from post-traumatic stress disorder. It was inadvertently used to his advantage when he began to sob real tears.
During the bar scene, where George prays, Stewart was so overcome with emotion, that he started crying. Frank Capra loved it so much, he had the footage re-framed and expanded the shot to capture the expression of James. This is why the image is so grainy in comparison to the rest of the movie.
According to Carol Coombs (Janie Bailey), Stewart was also very convincing at yelling at the children, that it made them genuinely scared enough to cry easily.