Billing Displacement: While Kris Kringle is definitely the story's central character, Edmund Gwenn was billed third behind Maureen O'Hara and John Payne in the original film. This wound up paying off for Gwenn, who was nominated for (and won) the Best Supporting Actor Oscar.
Creator Backlash: Subverted. Maureen O'Hara had to be forced to do the film by contract; she'd just arrived home in Ireland only to be told she had to fly back out to America to film it. But she ended up loving the script. According to her autobiography, everyone in the cast bonded very well.
Enforced Method Acting: Natalie Wood really did feel surprised when she tugged at Kris Kringle's beard, and saw him naturally react in minor pain.
Executive Meddling: The film was originally released in May despite being a Christmas film, due to an executive that believed people were more willing to see movies in May instead. This famously led to a marketing campaign that hid the Christmas themes of the film, including a trailer that instead of showing the film showed a fictional producer trying to figure out how to sell it while other contemporary stars raved about it. Fortunately, the film turned out to be a massive success anyway and actually ran long enough that it was still in theaters when Christmas actually rolled around.
Keep Circulating the Tapes: The 1973 made-for-TV remake has yet to receive a DVD release. However, Netflix did have it for streaming.
Playing Gertrude: Maureen O'Hara was 27 when the film came out and played the mother of a 8-year-old Natalie Wood.
Throw It In: Child actor Robert Hyatt, who played the District Attorney's son who gets questioned on the stand, came up with the "Daddy told me so" line, which was unscripted.
Unintentional Period Piece: Which leads to a case of Hollywood Law in the 1994 remake when it sticks too closely to the original's story, as after 1975 it would only be necessary to prove Kris wasn't a danger to himself or others, rather than entirely disproving that he was mentally ill.
The original title was 'Christmas Miracle on 34th Street'. The Christmas part was dropped when all references to the holiday were removed from the marketing campaign. The title of the script that Maureen O'Hara was first shown was 'The Big Heart'.
John Payne wanted really badly to make a sequel, and Maureen O'Hara claimed they had talked about it for years. He apparently wrote a screenplay for it but died before it could be produced.
Natalie Wood was asked to star in a TV remake with her daughter. She turned it down, not wanting her daughter to become a child star.
The 1994 remake:
Ability over Appearance: The original plan was to give a Gender Flip to Susan and have the remake's child be a boy. According to Mara Wilson's autobiography, a few people she had worked with recommended her for the part, and the character became a girl once again.
Actor Allusion: Mrs Walker stands in a department store looking down at the children from the second floor. The way the scene is done is a clear nod to Elizabeth Perkins's role in Big - and her hair is done the same style it was in the latter.
Creator Backlash: Zig-zagged. Mara Wilson is happy to have done the film, but she was annoyed about several changes made. Susan was written to be a funny, quirky child but during production was changed to become more Shirley Temple-like - emphasizing her cuteness. A large number of critics had a Tastes Like Diabetes reaction - which Mara herself agrees with.
The Danza: Middle name variety. Susan's middle name is Elizabeth, as is Mara Wilson's.
Harsher in Hindsight / Hilarious in Hindsight: The film's central theme is learning to have faith, which it conveys by drawing a parallel between the characters' faith in Kris and religious faith in God. However, Mara Wilson later became an atheist. Depending on your viewpoint, this could be seen as either one of these tropes.
Remake Cameo: Alvin Greeman, who played Alfred in the original, plays the doorman.