Dawson Casting: 38-year old James Stewart and 25-year-old Donna Reed trying to pass as teenagers. Justified in that they need to play older versions of their characters as well.
In a bit of Lampshade Hanging, Uncle Billy notes that "Nobody changes, here!" after Harry comes home from college.
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.
Hey, It's That Voice!: If you've seen the uncensored Tom and Jerry shorts, you might notice Ma Bailey's maid is played by the woman who voiced Mammy Two-Shoes, Lillian Randolph.
Also, Snow White (Adriana Caselotti) is now a singer at the martini bar, even though Caselotti didn't star in a lot of roles after Snow White and the Seven Dwarfs, as she didn't want to disillusion any fans who only saw her as Snow White.
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.
Screwed by the Lawyers: After Republic Pictures claimed rights to the soundtrack and The Greatest Gift, they put an end to the Christmastime tradition of this movie being shown on virtually every channel, at any time of day, by giving NBC exclusive broadcast rights. Now it only airs twice every December. (Although, if you find this movie more enjoyable in moderation, or wish you could avoid it altogether when watching TV at Christmastime, maybe "Screwed" sounds like too harsh a word.)
Throw It In: In the movie, Uncle Billy staggers away drunk from a party, you hear a crashing metal sound, and (offscreen) 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.
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.
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.