George hopes to go to college to "learn how to build things." In real life, James Stewart majored in architecture at Princeton University.
Lionel Barrymore played Scrooge in the popular annual radio version of A Christmas Carol, so casting him as Potter emphasized the similarities in the two characters for 1946 audiences.
The newspaper Bert the cop is holding when George asks Ernie the cab driver for a ride, has the headline "Smith Wins Nomination".
Actor-Shared Background: Both James Stewart and Donna Reed came from small towns; Stewart from Indiana, Pennsylvania, and Reed from Denison, Iowa. She demonstrated her rural roots by winning an impromptu bet with Lionel Barrymore when he challenged her to milk a cow on-set.
The Cast Showoff: For the scene that required Mary to throw a rock into the window of the Granville House, Frank Capra hired a marksman to shoot it out for her on cue. To everyone's amazement, Donna Reed broke the window with true aim and heft without the assistance of the hired marksman. Reed had played baseball in high school and had a strong throwing arm.
Creative Differences: Frank Capra disagreed with cinematographer Victor Milner and eventually had him replaced. Some of Milner's scenes were re-shot by Joseph Walker.
Creator Backlash: Two of the writers called the finished film "horrid" and refused to see it when it was released.
Reed ends up inverting the trope towards the end of the film. The age gap between George and Mary is only four years, as opposed to the thirteen year gap between Reed and Stewart. At the end of the film George's age roughly matches Stewart's, which means Mary is about ten years older than Reed.
In a bit of Lampshade Hanging, Uncle Billy notes that "Nobody changes, here!" after Harry comes home from college.
Made even more Hilarious in Hindsight by the fact that some 13 years later James Stewart will still play a romantic lead opposite 25-year-old actress.
Deleted Scene: Potter was originally supposed to die of a heart attack while counting his ill-gotten money, but Frank Capra decided it was too mean-spirited. The  is sadly lost.
Doing It for the Art: During filming of the scene at Martini's Bar, James Stewart was so overcome with emotion while praying that he began to shed manly tears for real. Seeing this, Capra had to reframe the shot in order to get it closer than was actually filmed because he wanted to catch the expression on Stewart's face. At the time this was an extremely time-consuming project, each frame had to be done individually, making it only one step away from stop-motion animation. Capra spent the whole night doing it.
Remember, too, that at one point Stewart had wanted to leave Hollywood behind after his World War II experiences. That Capra was able to woo him back into acting is this all over.
According to Robert J. Anderson, H.B. Warner really was drunk during the scene in which Mr. Gower slaps young George. Warner's slaps were real and caused real blood to come from Anderson's ear. After the scene was finished, Warner hugged and comforted Anderson.
The scene on the bridge where Clarence saves George was filmed on a back lot on a day where the temperature was 90 degrees Fahrenheit. This is why James Stewart is visibly sweating in a few scenes.
He Also Did: Sheldon Leonard, who played Nick-the-bartender ("Get me! I'm givin' out wings!") later became a television producer, where he was responsible for bringing some of the most famous shows ever into being: including The Dick Van Dyke Show and The Andy Griffith Show.
Irony as She Is Cast: George Bailey is denied from military service during World War II because of his deaf left ear. His little brother is accepted and becomes a medal winning pilot. The real Stewart was a bomber pilot and Lt. Colonel in WWII, later being promoted to Brigadier General after the war. In fact, he would continue to serve in the Reserves until he retired from the military several decades later.
Money, Dear Boy: Sheldon Leonard said in an interview that the only reason he agreed to play Nick the bartender in this film was so that he would have money to buy baseball tickets.
Release Date Change: The movie was originally slated for 1947 release, but when Technicolor was unable to deliver prints in time for RKO's Christmastime 1946 release of Sinbad, the Sailor, it was rushed into theaters. The titles were not reshot, and thus bear a 1947 copyright.
After Uncle Billy staggers away drunk from a party, you hear a crashing metal sound, and he calls back, "I'm all right! I'm alllllllll right!". We presume that Uncle Billy stumbled into some garbage cans. In fact, the crashing noise was from a stagehand dropping equipment, and Thomas Mitchell's "I'm all right!" was an ad-lib. The crash, Mitchell's line, and Jimmy Stewart's subsequent onscreen corpsing were all kept in, and the stagehand got a bonus.
The first kiss with Mary, Jimmy Stewart was nervous about his first onscreen kiss since coming back from World War II. It was filmed in one unrehearsed take and part of the kiss had to be cut due to being too passionate for the censors.
From the same scene, there were more lines that George was going to say to Mary before kissing her, but Stewart essentially pulled a "Shut Up" Kisson himself and just cut ahead to the act of passion.
In the scene when there's a run on the Savings & Loan, Ellen Corby (in her only scene in the film) threw in her request for a very specific amount of money, not just her entire savings back; Stewart was so amused that his reaction is genuine.
Vindicated by Cable: It was not a big hit on initial release. It actually became a public domain title in 1974, so virtually every TV station around started airing it around Christmas due to it being so inexpensive. Since the early 1990s its copyright has been re-established (see the other wiki for all the details), but it's still a holiday staple, albeit exclusive to one broadcaster.
It was obscure enough in 1968 that Andrew Sarris doesn't even mention it in the Frank Capra entry for his landmark Auteur Theory book The American Cinema. These days it's usually considered Capra's best work.
Jean Arthur was Frank Capra's first choice for the part of Mary Bailey. However, she declined the role since she was already committed to a Broadway play. Ginger Rogers was offered the role, but turned it down, thinking the part was too small. Olivia de Havilland was also considered.
In the original script, Clarence confronts Potter about what he did to George. It was to take place right after Potter yelled, "And Happy New Year to you, in jail!" Since the scene involved a terrified Potter having a heart attack upon learning that he's going to Hell when he dies, it was deemed too grim and cut out.
A number of alternative endings were considered, with Capra's first script having George fall to his knees saying The Lord's Prayer (the script called for an opening scene with the townspeople in prayer). Feeling an overly religious tone didn't have the emotional impact of family and friends coming to George's rescue, the closing scenes were rewritten.
The film was originally going to end with everyone singing "Ode to Joy".
Write What You Know: Sam makes a fortune in plastics while Harry becomes an engineer at his father-in-law's glass factory. Both of these come out of Frank Capra's own education in chemical engineering; ironically, Capra himself was unable to find a job with his background and, like George Bailey, considered himself a failure for many years.
Write Who You Know: The Martinis were based on Frank Capra's own family, who emigrated from Sicily in 1903. A goat accompanies them in their car; "capra" means goat in Italian.