Accidentally Accurate: A much-criticized scene had Jayne place his beloved rifle Vera in a spacesuit in order to fire in space, with the given reason that it needs oxygen to fire. Bullet propellants contain all that's needed for combustion, meaning that normal guns should be able to fire in the airless environment of space. However, there actually is a valid reason for putting an atmosphere around it: exposure to hard vacuum can cause many types of non-specialized lubrication to flash-evaporate and render the firing mechanism inoperable, meaning the gun would not even fire in the first place.
Built with LEGO: Credit must be given to the fan who built a minifig-scale model of Serenity, which is over seven feet long. You can also buy a custom kit to make your own (smaller) version, though LEGO themselves turned down an offer to make the set official.
Executive Meddling/Screwed by the Network: Fox insisted that Joss Whedon to write a second pilot because they wanted more action and less drama. They also threatened to pan-and-scan crop, no matter how it was shot, necessitating re-shoots. Then they aired the episodes out of order and preempted a bunch of them for baseball. The series didn't even get to finish its first season.
Reality Subtext: While filming the funeral scene for "The Message," the crew was informed that the show had just been canceled. The sadness you see on their faces is real. This was also what the composer Greg Edmonson had in mind when he wrote the music for that particular scene.
The Other Marty: Rebecca Gayhart as Inara, who was recast after filming a few scenes of the pilot. Joss Whedon does not fault her performance itself, but he apparently foresaw a problem since he shot most of her scenes in single-person profile so he would not need to bring in other actors in the event that he had to re-shoot her scenes.
Throw It In: One scene at the end of the episode "Ariel" has Mal debriefing the crew after retrieving River, Simon and Jayne. When Mal walks up to Kaylee, he reaches for her shoulders, spins her around and pulls her close to him. This was apparently improvised by Nathan Fillion, and the writers decided to keep the scene.
According to Tim Minear in the Firefly 10th anniversary special "Browncoats Unite", the part at the end of "The Message" where Kaylee holds Simon's hand was not in Joss Whedon's original script.
Alan Tudyk got the script for the scene where Mal and Wash argue in Out of Gas about an hour before shooting, and was furious that he had so much technobabble to memorize on such short notice. He and Minear immediately decided that Wash should be furious in the scene.
Airing on the Science Channel in November 2012, the Firefly 10th anniversary special "Browncoats Unite", which featured interviews with most of the cast and two of the writers, Tim Minear and Jose Molina, brought up various concepts for potential episodes:
Tim Minear elaborated on the vial and syringe Inara was seen with in the pilot episode "Serenity", explaining that it was a drug which, if she were raped, would cause the rapist to die a horrible death. Inara would have been kidnapped by Reavers, and the crew would track her down. When Mal enters the Reaver ship, he finds all the Reavers dead. He would then see Inara after she has been horribly brutalized, take her hand and treat her like a lady. According to Minear, this was one of Joss Whedon's first ideas regarding the kinds of stories they would tell through the series.
Alan Tudyk, Gina Torres and Sean Maher imagined the couples of the show (Wash/Zoe and Simon/Kaylee) having children and living onboard Serenity, with Zoe arguing with Jayne over who's going to teach the kids about guns.
Alan Tudyk also had a concept where they would transport feral dogs for dogfighting, and then River would commune with the dogs and actually tame them, rendering them useless.
Adam Baldwin came up with a scenario where Jayne would get his own ship and try to compete with Mal, fail badly and end up returning to Serenity a bit more humbled.
Nathan Fillion pitched an elaborate story with a moral dilemma that he heard from Joss Whedon, where the crew would land on a planet and would be treated very well, before learning that the planet was dying and the inhabitants want Mal to help them escape. However the planet is so distant that if they take on refugees they will run out of air, unless they meet into another ship. Mal pretends to agree, and while the crew are sleeping, takes control of Serenity and flees the planet. As they escape, they never meet another ship, and realize if they saved the people they all would have died. Mal then assumes responsibility for their actions, emphasizing that what happened was his fault alone.
In the Comic-Con panel for Firefly's 10th anniversary, which included Joss Whedon himself, an audience member asked Joss how the series ending would have differed from Serenity had the first season been confirmed as the only season of Firefly. Joss replied that he would not have killed anyone, and that the back-stories of Inara and Book would have been further elaborated.
According to a 2013 interview with Joss Whedon "“I had planned to do an episode of Firefly with Amy and Alexis and James Marsters as part of a travelling Shakespeare troupe. Because it’s sort of a staple of the John Ford Westerns, there’s always that over-the-top theatre guy. And I thought it would be terrific to have them to try and put on a play in the cargo bay.”
The RPG includes a plot that would have been part of the second season, that the Alliance is on the brink of economic collapse and trying very hard to prevent this by covering it up.
Word of Saint Paul: In a commentary, Alan Tudyk gives a semi-serious speculation on what Wash was up to during the Unification War: he got a job ferrying supplies (for which side isn't clear), but was shot down on his first mission and spent the rest of the war in prison, where he survived Scherazade-like with his puppet shows. Many fans added this to their Fanon.
Joss Whedon had cast Summer Glau before Firefly - she originally appeared as a Prima Ballerina in the Angel (1999) episode "Waiting in the Wings". Whedon remembered her from this and contacted her about the part of River.
Chinese phrases are often inserted into the English dialogue (sometimes to get away with saying things that the censors wouldn't allow them to say), and the ship has some signs in Chinese.
In the original conception of the show, Joss Whedon was only going to have five main characters on the ship. Throughout the development process, the character list got upped to nine.
The entire interior of Serenity (and some of its exterior) was built full-scale and almost completely contiguous, split across two soundstages - one stage for the upper deck and one for the lower deck, shuttle docks and hold.
The cast preferred to use the ship's lounge instead of a green room or other accommodations when waiting between shots.
The character 'Captain Malcolm "Mal" Reynolds' was ranked #18 in TV Guide's list of the "25 Greatest Sci-Fi Legends" (1 August 2004 issue).
The Alliance's full title is the "Anglo-Sino Alliance". Joss Whedon intended the Alliance to be the merger of the USA and China, the last of the world's superpowers. The Alliance flag, seen in the original pilot episode, is a blending of the US and Chinese flags.
Joss Whedon originally thought up the premise of the show with his friend Tyler Lovelly, who was not interested in pitching the show, so only Whedon was involved in production.
Joss Whedon has said that the original inspiration for the project came to him after he finished reading Michael Shaara's "The Killer Angels", a historical-fiction novel about the Battle of Gettysburg.
Jubal Early was a Confederate general who played a role in the Battle of Gettysburg. Nathan Fillion claims him as an ancestor.
Many of the names of off-camera and minor characters are drawn from the ranks of science fiction writers. Notably, Bester (Alfred Bester) as the original mechanic of Serenity and Brennert (Alan Brennert) and Ellison (Harlan Ellison).
The cast had a running gag where they would yell Summer Glau's name whenever they flubbed a line or messed up. It began after she forgot her line at the end of a particularly difficult scene. (The scene in question was The Oner at the end of "Objects in Space".) The gag continued through the filming of Serenity (2005).
All the Alliance officer and soldier uniforms are leftovers from Starship Troopers (1997).
Some of the weapons used in the series were present-day weapons, which were picked based on their somewhat futuristic look. No modifications were made by the prop department to either disguise them or make them more look more futuristic. Alliance soldiers are seen carrying Enfield L85 rifles, which have been standard issue in the British Army since the early 1980s. The Browncoats are seen using Heckler & Koch G36 rifles, which are standard issue in the German Army.
In several episodes (for example "The Train Job"), Jayne wears a German police jacket (green, military style), from the federal state of Rhineland-Palatinate (German: Rheinland-Pfalz). This can be recognized by the badge on the right sleeve with the word "Polizei" (German for police) on the top and the emblem of Rhineland-Palatinate underneath.
The lever-action Winchester carbine carried by Zoe (Gina Torres) throughout the series is an example of the Mare's Leg, a weapon used by Steve McQueen in the 1960's television western series "Wanted: Dead or Alive." Torres' exact prop was the one used on The Adventures of Brisco County, Jr..
This same weapon is used by Woody Harrelson's character Tallahassee in Zombieland
Jayne's favorite gun, Vera, is a 12-gauge Saiga shotgun, originally modified for the film Showtime.
The DVDs of Firefly were flown up to the International Space Station by astronaut Steven Swanson on board the shuttle Atlantis during its June 2007 STS-117 mission.
Every scene in space is shot without sound effects of the ship moving, precisely as it would in real vacuum. No air, no sound. (Note this does not cover dramatic music that plays during space scenes.)
The sound made by Mal's hand gun is the same sound as the eponymous character's hand gun in the Spaghetti Western Django (1966), played by Franco Nero.
The Chinese most commonly used throughout the series is the most widely used dialect of Chinese - Mandarin - however, in "Out of Gas" the Chinese used over the Life Support Klaxon warning is the Cantonese dialect, a lesser-known dialect but still widely used (just to a smaller extent than Mandarin).
The "Crazy Ivan" maneuver pulled by Wash in the pilot is inspired by a nearly-identical submarine maneuver. It was used during the early days of the Cold War by Russian submarines, in order to check the "baffle", the sonar dead zone directly behind the submarine; and given the name "Crazy Ivan" by US and allied submarine crews. The adoption of towed sonar arrays effectively ended the necessity for the "Crazy Ivan" maneuver. Referenced in its original context in The Hunt for Red October.