I knew, I knew!
I don't know if it's in the biography but I'll relate a story I heard him tell to Johnny Carson.
When he was in his late teens he had a job at a hardware store and lived in a boarding house. His room was on the side of the house and he could see the edge of the front porch of the neighbor's house. Every night for many weeks a cat would sit on the neighbor's porch and "meow" for hours at a time. Eventually, Jim borrowed a take down .22 from the store and secreted it up to his room. Very late at night when the cat was howling he was preparing to shoot it from his windows when suddenly the neighbor's lights came on. The neighbor appeared on the porch in a bathrobe and stood next to the howling cat. The neighbor seemed to be looking around for something while he he had a smoked. (The cat continued to "meow").
Jim said he was petrified that the neighbor had seen him and that the police were coming to arrest him. After a few minutes the neighbor put out his cigarette, reached into his bathrobe pocket, pulled out a pistol, shot the cat and went back into the house.
One very tense, but relieved James Stewart went back to bed.