For the scene that required Donna Reed to throw a rock through the window of the Granville house, director Frank Capra hired a marksman to shoot it out on cue. To everyone's amazement, Reed broke the window by herself. She had played baseball in high school and had a strong throwing arm.
As Uncle Billy drunkenly leaves the Bailey home, it sounds as if he stumbles into some trash cans on the sidewalk. In fact, a crew member dropped a large tray of props right after Thomas Mitchell went off-screen. James Stewart began laughing, and Mitchell quickly improvised, "I'm all right! I'm okay!" Director Frank Capra decided to use this take in the final cut and gave the stagehand a $10 bonus for "improving the sound."
During filming, James Stewart was actively suffering from PTSD and depression due to his service in World War II. Stewart told friends that he related a lot to the character of George Bailey, some of the scenes where George acted out in anger was considered "cathartic" for Stewart and his struggles with mental health. Stewart's acting became notably darker after the war as he knew he had killed civilians in Germany and occupied Europe.
James Stewart was nervous about the phone kiss scene because it was his first onscreen kiss since his return to Hollywood after the war. Under director Frank Capra's watchful eye, Stewart filmed the scene in only one unrehearsed take, and it worked so well that part of the embrace was cut because it was too passionate to pass the censors.
The gym floor that opens in the middle to reveal the swimming pool underneath was filmed at Beverly Hills High School In Beverly Hills, California, USA was real and is still in regular use. The same gymnasium moving floor was used in a similar school dance scene in Whatever It Takes (2000), fifty-four years later. The young man who opened the floor to the pool was Carl 'Alfalfa' Switzer in an uncredited role. He was one of the stars in the Our Gang 'Little Rascals' series.
Frank Capra: [Warner] One of the reasons H.B. Warner got the part of the pharmacist, Mr. Gower, was that he actually studied medicine before going into acting. He was also in some of Frank Capra's other films, including Mr. Deeds Goes to Town (1936), Lost Horizon (1937), You Can't Take It with You (1938) and Mr. Smith Goes to Washington (1939). The character's name "Gower" derived from Capra's employer Columbia Pictures, which was located on Gower Street for many years. Also on Gower Street was a drugstore that was a favorite for the studio's employees.