He was the first movie star to enter the service for World War II, joining a year before Pearl Harbor was bombed. He was initially refused entry into the Air Force because he weighed 5 pounds less than the required 148 pounds, but he talked the recruitment officer into ignoring the test.
He graduated from Princeton with a degree in architecture.
When he won the Best Actor Oscar in 1940, he sent it to his father in Pennsylvania, who set it in his hardware shop. The trophy remained there for 25 years.
He played the accordion.
He never took an acting lesson, and felt that people could learn more when actually working rather than studying the craft.
While always gracious with his fans, he was always very protective of his privacy. A notable example of this occurred when a nervy family of tourists set up a picnic on his front lawn. He came out of his house and, without uttering a word, turned on the sprinklers.
His best friend was probably Henry Fonda, whom he met while at acting camp. After each married and settled down, their children noted that their favorite activity when not working seemed to be silently painting model airplanes together.