There Were More Firefly Guest-Stars On Castle Than You Probably Thought

Out of all the biggest mistakes which have ever been made in the entertainment industry, mistreating and prematurely canceling "Firefly" is at least in the top five (if not higher). Naturally, the talented actors who made up the main cast have moved on to bigger and equally awesome things, like Morena Baccarin starring in "The Endgame" and Alan Tudyk with a leading role in "Resident Alien." Arguably, one of the most successful post-"Firefly" career moves happened for Nathan Fillion (who plays Captain Malcolm Reynolds on "Firefly") when the space cowboy hung up his space pistols and traded them in for a writer's pen on ABC's campy crime drama "Castle." He plays titular writer Richard Castle, who gets to work alongside NYPD detectives in order to glean inspiration for his murder mystery books.

When Fillion moved over to ABC, though, he didn't go alone. And his love of the sci-fi hit was still as fresh in his heart as it was for the fans. Not only were there several fun little "Firefly" easter eggs sprinkled in for fans, but many of his "Firefly" alumni made guest appearances on the show. Actors Richard Burgi, Fredric Lehne, and Gregg Henry each had one-episode guest appearances on both "Firefly" and "Castle." But there were some even bigger "Firefly" stars that paid Fillion a visit on his new show, to the delight of fans.

In "Firefly," Gina Torres plays Corporal Zoë Alleyne/First Mate Zoë Washburne, a war buddy of Reynolds who eventually falls for their new pilot, Hoban "Wash" Washburne (Tudyk). She's as fiercely protective of her husband as she is effective in combat, turning the old "damsel in distress" trope right on its head. And she channeled some similar energy when she guest-starred on "Castle."

In Season 5, Episode 14 of "Castle" (titled "Reality Star Struck"), Torres plays reality TV star and fashion icon Penelope Foster, who gets blamed for the murder of one of her co-stars. She looks really guilty at first, especially since the murder victim was having an affair with her husband, and she has a broken nail which matches the victim's defensive wounds. But like most murder mysteries, it's never who you first suspect. She's eventually vindicated once Castle and the police discover that the affair was faked for TV ratings and that she had a solid alibi for the night of the murder. 

Back during her "Firefly" days, Jewel Staite played the adorable ship mechanic Kaylee Frye. She was as sweet and naive as she was in love with frilly dresses and strawberries. Her character has been a fan favorite for many years. She's enjoyed a fruitful career since, including an untitled sequel to "Quantum Leap" (via IMDb). But she also guest-starred on an episode of "Castle" back in the day.

It was Season 8, Episode 20 — "Much Ado About Murder." Castle and the detectives must try and figure out who brutally murdered a high-profile actor who stepped away from the silver screen to try his hand at the theater. Staite plays the director of the play, Erin Cherloff. She seems innocent at first, but once the truth comes out — that the murder victim stole an Oscar-worthy directing job right out from under her — she tells the detectives that she snapped. And they promptly snap some handcuffs on her wrists.

Summer Glau played one of the most interesting and complex characters on "Firefly." She was River Tam, a young girl whose intelligence, telepathic abilities, and combat prowess get her kidnapped by the corrupt Alliance government and experimented on. The trauma she endures in custody leaves her mental health in an unsteady state, creating situations in which she depends on the Firefly crew's protection about as often as she saves their bacon in return.

In Season 8, Episode 14 of "Castle," known as "The G.D.S.," Castle is in a rush to solve a murder before his competition so that he can earn a spot on the secret and distinguished Greatest Detectives Society. The competition he's up against is private detective Kendall Frost, played by Glau. Towards the end of the episode, even though he ends up catching the serial killers and earning his spot, he declines at the last minute with high praise for Frost, saying she should get the seat instead.

In "Firefly," Adam Baldwin plays Jayne Cobb, a space mercenary who the viewers aren't immediately certain that they can trust. After all, he earns his spot on Mal's ship after holding him at gunpoint and turning on his business partners once Mal offers him more money. Throughout the course of his time on the ship, though, he discovers his buried morality and eventually turns over a new leaf.

Baldwin appeared on not just one, but two episodes of "Castle." The first appearance was in Season 4, Episode 21, "Headhunters." He plays Detective Ethan Slaughter, a tough-as-nails, loose-cannon rogue cop stereotype in diametric opposition to Detective Beckett. Castle and Beckett save him from himself after he coerces a false murder confession out of an innocent kid. The two solve his murder case for him and put a dangerous mob boss behind bars in the process.

The man named Jayne came back for a second episode in Season 8, Episode 6, "Cool Boys." Things start out hot and heavy when Slaughter tracks Castle down to help him solve a murder which he is accused of committing. Castle secretly agrees to help him, betraying his friends at the NYPD. But all is forgiven once the real killer is found and justice gets done.