Trivia: You may have noticed that the man who tries to pull a gun during the opening bank robbery and is later killed out of mercy by Mal looks familiar. It is none other than Glenn Howerton, best known for playing Dennis on the dark comedy series "It's Always Sunny in Philadelphia." This movie was released just a few weeks after "Always Sunny" first premiered.
Trivia: The date September 20th has some relevance for this franchise. Mal's birthday in this film is given as September 20th of the year 2468. This is a bit of an Easter Egg reference to former "Firefly" staff writer Ben Edlund, who was born on September 20th in the year 1968. (Same day, exactly 500 years apart.) And on top of that, in a bizarre coincidence, "Firefly" also premiered on September 20th in the year 2002.
Trivia: Despite "Serenity" being the last live-action entry in the "Firefly" franchise, creator Joss Whedon eventually continued the story in a series of comic-books, which are considered canonical with the show and this film. This includes a prequel delving into Shepherd Book's backstory, and two direct sequels ("Serenity: Leaves on the Wind" and "Serenity: No Power in the Verse"), which were released in 2014 and 2016 respectively.
Trivia: Despite being well-known for killing-off his characters, director Joss Whedon originally didn't plan to kill-of the characters Wash or Shepherd Book. He only chose to because the actors were unable to contractually commit to any potential sequels at the time due to possible scheduling conflicts. Whedon later stated that he would bring both characters back into the franchise if any sequels/follow-ups are made.
Trivia: The child actress who portrays the young River in dream sequences had her voice dubbed over by Summer Glau, who portrays the "older" River. This was done so that audiences could more easily realize the child is River. (And thankfully, Glau's voice worked for the child actress). The child actress portraying the younger River was reportedly heartbroken that her voice had been dubbed over.
Trivia: The "one-shot" sequence in the beginning of the film that introduces the crew of the Serenity and the interior of the ship was actually two shots cleverly spliced together, for both practicality purposes (as one mistake could ruin the entire 4-minute scene) and also because there wasn't enough studio-space to build the entire interior of the Serenity as one unbroken set. Eagle-eyed viewers may be able to notice the moment of the splice, right after Mal runs into Simon and they start to descend the stairs. The splice takes place when the camera does a small whip pan from right-to-left.
Trivia: The man who opens the safe at the beginning of the film was played by Weston Nathanson... however, his voice was provided by director Joss Whedon. As is often the case, Nathanson's lines had to be re-recorded in post-production, but Nathanson wasn't available. Joss stepped in and dubbed the lines himself.
Trivia: Universal originally estimated the budget of the film at $100 million, given the size of the movie and the amount of visual effects. Director Joss Whedon told the producers that he could make the movie for under $40 million - true to his word, the film's final budget was a relatively tiny (by action-movie standards) $39 million.
Trivia: "Serenity" has the somewhat dubious honor of being one of the first four films that were released on the ill-fated HD-DVD format on April 18th, 2006. (It was finally released on the competing Blu-Ray format two years later after HD-DVD was discontinued.) Even more dubious, "Serenity" was also the first HD-DVD film to be pirated on BitTorrent.
Trivia: Some Spoilers. Around the time "Serenity" was released in theaters, Joss Whedon co-wrote a comic-book entitled "Those Left Behind" that explains the events that occurred between the end of "Firefly" and the start of this film, and answered some leftover questions from the series. In the comic, among other things, the "Blue Hands" finally make their move and attack Serenity, but are eventually thwarted and killed when they are incinerated by Serenity's thrusters, leading the alliance to give their mission to the much more dangerous "Operative." We also learn that Shepherd Book left Serenity after Mal pushed him too far, making him punch Mal in the face. This finally makes him realise that his time on Serenity has hardened him too much and lead him astray from his attempts at a peaceful life, and he decides to leave on good terms with the crew.
Trivia: DVD copies of the TV show "Firefly" and this follow-up movie "Serenity" were flown to the international space station in 2007. Several members of NASA are big fans of the show, and had the DVD's (along with some "Firefly" themed gear) sent into space for crew-members to watch. Late on in 2014, astronaut Steve Swanson (who was the one that originally brought along the DVD's seven years prior) also took the first official Instagram photo from space whilst wearing a "Serenity" T-shirt.
Trivia: In a nice bit of continuity, if you pay close attention, you can see some small scars on Mal's body a few times in the film. Most of them correlate to injuries he received earlier in the series "Firefly." (eg. You can see the scar from when he was shot in the abdomen in the episode "Out of Gas" if you look closely).