Ability over Appearance: Bong Joon-ho had reservations about casting Chris Evans as Curtis Everett, because of his muscular physique. He felt that as a resident of the extremely poverty-stricken tail section, Curtis should not be especially physically fit. Costuming and careful camera angles kept Evans' physique from showing.
Ascended Fanon: In a way, the movie can be interpreted as an Elsewhere Fic to the comics, which present the possibility of there being multiple Snowpiercers instead of just the two. Terminus officially confirms that the movie takes place on a third Snowpiercer, and that there were as many as ten roaming the earth at some point.
Bong Joon-ho came across the original comic in his local comic book store and found himself so engrossed in the story that he read the entire series right then and there. Upon finishing the comic he was hell-bent on making a cinematic adaptation.
Many of the cast also signed on, despite the project being a relatively low-budget foreign film, because they were intrigued by the story and because Bong gave them significant leeway on interpreting their characters.
The film saw a very limited domestic release in Europe, reaching only a handful of countries like France and Norway. If you have any problems understanding English (notably spoken in a wide range of American, British, and European accents) and/or can't speak Korean, enjoy deciphering the one subtitle track (which depends on which country the print is for) and missing Namgoong's ending monologue, or prepare to pay an inflated price to ship an American copy.
Dyeing for Your Art: Chris Evans grew a beard for his role as Curtis, and had to use a face mask when he filmed The Stinger after the ending credits of The Avengers. Note how he keeps his chin (and thus his beard) in his hands as Captain America in the scene. However, he didn't have time to lose weight for this film so the costume designers had to spend an unexpectedly large portion of their small budget figuring out ways to hide his muscles since Curtis is supposed to be suffering from years of inadequate nutrition.
Executive Meddling: Attempted by The Weinstein Company. They're infamous for acquiring foreign films, cutting them down and promptly "americanizing" them. When they tried to cut up to 20 minutes from the Korean version, they were hit with bad publicity from cinephiles. Along with critical acclaim towards Joon-Ho's cut, this led the Weinsteins to back off.
Fake Nationality: Given by his name and the fact that he says "Chikusho!"note Which roughly translates to "Shit!" right before Grey kills him, Officer Fuyu is of Japanese descent. His actor, Steve Park, is American of Korean descent.
I Knew It!: The film essentially has nothing in common with the comics aside from the setting... which, given the openness of the endings, makes it fairly open to potentially being set elsewhere in the universe of the comics. As of Snowpiercer 3: Terminus, the film is confirmed to be canon to the comics.
One-Book Author: Emma Levie, who plays Claude, had starred in one film in the Netherlands, the dark drama Lena, before this. Snowpiercer has been her last film to date. Her primary career is as a painter.
The only place where the original comic was published outside of France was South Korea. Also, the original authors had previously refused an offer to turn it into a film.
There were some planned concepts that had to be cut due to budget restrictions, such as:
The Engine having more than one-hundred children as the workers.
An additional zoo section being shown after the aquarium section, with a scene of a bent-neck giraffe being shown.
A brothel section with tail-section women in it, which originally appeared in the comic.
According to this 2014 Screen Craft interview, Curtis was originally going to have a Love Interest who went to the front section some years prior to the events of the movie, and his motivation to reach the Engine would have partially revolved around finding her.
The ending was originally planned to have a narration by Tanya's son Timmy, but as an adult, indicating that he and possibly Yona survived long after the events of the movie. This was cut because Bong Joon-ho wanted the ending to be more open.
The Cast Showoff: That really is Sam Otto playing that stunning version of "Winter Song" in episode 9.
Channel Hop: The series did it twice before it even premiered. It was originally announced as a TNT original before a move to TBS was announced, albeit to much confusion because the show's dark nature clashes with TBS' lighter tone. This was followed by a move back to TNT.
Sports Preemption: The show's much-hyped debut on TNT was delayed by half an hour by a golf game, to the outrage of fans whose TiVo recordings missed half the showing. Fortunately, it was replayed repeatedly throughout the week to follow, and prior to the second episode's debut a week later.
Troubled Production: The series was ordered to pilot in 2017, but Creative Differences between the showrunners and the network executives lead to the original pilot script being thrown out and caused production to be delayed for several years. The release of the series was further delayed, to the point that TNT ordered a second series before the first one even premiered in order to avoid losing the actors. And then filming on Season 2 got shut down by the COVID-19 Pandemic. This, on top of the sheer logistical challenge of filming action sequences and crowd scenes on sets crammed into spaces the size of train cars.
What Could Have Been: Many characters were radically different when originally cast. Ruth and L.J. were originally third class passengers (a manicurist with a husband and son, and a midwestern greenhouse worker respectively), and Josie had a sick husband.