Trivia: Nathan Fillion, Gina Torres, Adam Baldwin, and Alan Tudyk all later played villains on Joss Whedon's other Mutant Enemy productions: Fillion appeared as Caleb in the final five episodes of "Buffy the Vampire Slayer," while Torres played Jasmine in season four of "Angel" and Baldwin played Marcus Hamilton in season five. Tudyk played Alpha in "Dollhouse".
Trivia: Nathan Fillion would hide snacks and drinks on-set (usually in crevices, drawers or just off camera), so they would be readily at hand in-between takes. After learning the series was canceled, he alleges that he purposely hid a half-finished drink in one of the pieces of the set, and never retrieved it, to see if anyone would eventually stumble onto it. He reportedly does not know what became of it, so there's a chance it's still in the set-piece in storage somewhere.
Trivia: Creator Joss Whedon, who is infamous for both killing off his characters and also having characters in relationships "break up", stated that if the series went ahead as planned, Wash and Zoe would have been the only couple in one of his series that would last from the start to the end, because he thought they were perfect together and shouldn't separate.
Trivia: Co-star Summer Glau reportedly flubbed a key line during filming one day at the end of a rather long take, and in frustration, one of the other cast-members (both annoyed but also amused by the flub) exclaimed "Summer!" Everyone on-set found this exclamation so funny, that they would all scream "Summer!" whenever they flubbed a line. Glau was reportedly annoyed by this at first, but later began to find it amusing like the others.
Trivia: If the show wasn't canceled, the contents of the mysterious syringe Inara has in the pilot would have been revealed. It was set to contain a very potent strand of an artificially created STD - non-fatal to Inara, but fatal to anyone who had sexual contact with her within a certain span of time after she injected it. In a planned storyline, Inara would inject herself with it shortly before being captured by a Reaver ship. After the crew of the Firefly breaks onto the Reaver ship to rescue her, they'd find every single Reaver dead, implying that Inara had been assaulted in some way and/or raped by every single one of them. This would then lead into an ongoing character-arc regarding her depression over the assault.
Trivia: Joss Whedon had intended for the series to last seven seasons, and had even come up with plans for the various main season-storylines before the series was canceled. The follow-up movie "Serenity" features a modified and shortened version of what would have been the central storyline for seasons two and three. In addition, "Serenity" also features most of the revelations about River's backstory and abilities that were planned for the rest of the series, such as her being trained to be a living weapon, her unknowingly having learned Alliance secrets, her knowledge of Miranda, her overt psychic abilities, etc.
Trivia: At first Nathan Fillion insisted on doing all his own stunts. However, he discovered his stunt double wasn't being paid since he wasn't technically working, with Fillion doing all the stunts himself. Fillion felt bad for having taken his work, so he began using his stunt double from then on.
Trivia: Had the show continued, Joss Whedon contemplated giving James Marsters a cameo in an episode reprising his role of Spike from "Buffy the Vampire Slayer" and "Angel." It would have been a very quick "blink-and-you'll-miss-it" part, showing Spike sitting at a bar drinking and telling Mal "Nothing ever changes..." as he passes by.
Trivia: Executives at Fox took issue with nearly every aspect of the series during production, and caused a great deal of trouble for the creators. For example, Fox attempted to meddle with the story and characters, including attempting to force creator Joss Whedon to rewrite the characters Wash and Zoe so they wouldn't be married. Fox also disliked several of the episodes and aired them completely out of order (including airing the pilot last), making the plot extremely difficult to follow. Episodes were also frequently delayed for sporting events and shifted around, making it hard for fans to watch the series. Finally, Fox attempted to deceptively market the series as a "wacky" and "twisted" sci-fi comedy as opposed to marketing it as the sci-fi drama with comedic elements that it actually was. These are often cited among the many factors that contributed to its early cancellation. However, despite these factors, the series ended up becoming a massive cult hit among sci-fi fans.