Typically, the introduction to a new television series comes via its pilot episode. But the first episode that audiences see isn't necessarily the first episode that was made. Whether it was a case of networks tweaking the content, showrunners changing the cast, or something else entirely, here are 12 pilots that never made it to air.

1. Sherlock

In 2009, TV producers Mark Gatiss and Steven Moffat produced a 55-minute pilot of a modern-day version of Sherlock Holmes to gauge the interest level in a new TV series starring Benedict Cumberbatch as the titular detective and Martin Freeman as his assistant, Doctor John Watson. The BBC loved it, but wanted to expand the pilot to 90 minutes, with plans to air three 90-minute episodes in 2010. The writers expanded the script and the crew and stars re-shot the action, but most of the unaired pilot's dialogue was retained in Sherlock's first official episode, "A Study in Pink," a loose adaptation of Sir Arthur Conan Doyle's first Sherlock Holmes novel, A Study in Scarlet. The unaired pilot is a bonus feature on the Sherlock Season One DVD set.

2. Gilligan's Island

Between 1964 and 1981, Gilligan's Island aired 98 episodes over three seasons plus countless made-for-TV movies and spinoffs—but its original TV pilot was never shown on broadcast television. The show's creator, Sherwood Schwartz, made the pilot episode, "Marooned," to sell CBS on a new sitcom. Network executives loved Gilligan's Island, but wanted different actors in some of the key roles.

Russell Johnson (The Professor), Tina Louise (Ginger), and Dawn Wells (Mary Ann) weren't featured in the pilot episode; instead, John Gabriel played a high school teacher; Kit Smythe and Nancy McCarthy—who played Ginger and Bunny, respectively—were secretaries, not a movie star and/or farm girl. Bob Denver, Alan Hale Jr., Jim Backus, and Natalie Schafer still appeared in the original pilot episode as Gilligan, The Skipper, and Thurston and Lovey Howell. Elements of the original TV pilot were used as flashback sequences in Gilligan's Island's first episode, "Two on a Raft," which aired on CBS on September 26, 1964.

Another notable difference between the pilot and the first official episode was its theme song, which wasn't the memorable "The Ballad of Gilligan’s Isle." Instead, Star Wars film composer John Williams wrote a Calypso-style song that was used to tell the story of how the tourists were shipwrecked on a deserted island.

"Marooned" is a bonus feature on the Gilligan's Island Season One DVD set.

3. Game of Thrones

In 2009, two years before it premiered on HBO, TV producers David Benioff and D.B. Weiss acquired the TV rights to George R.R. Martin's Game of Thrones book series. Benioff and Weiss worked with director Tom McCarthy to make a pilot episode for HBO, which the premium cable network never aired because Benioff and Weiss felt that it was disjointed. Benioff and Weiss ditched McCarthy and hired TV veteran Timothy Van Patten to direct another pilot for HBO.

"I just didn’t feel connected to it," McCarthy told The A.V. Club in 2011. "It felt like more of it was [Van Patten's] than mine in terms of what you see on the screen now, and I think if you would talk to [Benioff and Weiss], they would say I was helpful in a lot of the process, but it certainly doesn’t feel like mine."

Benioff and Weiss re-wrote the script and re-cast a few of the key roles, including Catelyn Stark and Daenerys Targaryen. Originally, Jennifer Ehle and Tamzin Merchant played Catelyn and Daenerys before Michelle Fairley and Emilia Clarke were re-cast in the roles, respectively. The revamped Game of Thrones pilot, which reportedly cost between $5 and $10 million to produce, aired on HBO in 2011.   

4. The Big Bang Theory

CBS passed on the original pilot for The Big Bang Theory, but liked the idea enough to order another version of the pilot from producers Chuck Lorre and Bill Prady. The pair kept the sitcom's main concept, but tweaked its characters and changed its cast.

Johnny Galecki and Jim Parsons still played Leonard and Sheldon, respectively, but producers ditched the two female leads: Gilda, a scientist colleague played by Iris Bahr, and Katie, "a street-hardened, tough-as-nails woman with a vulnerable interior," played by Amanda Walsh. Instead, a new character named Penny—Sheldon and Leonard's neighbor, played by Kaley Cuoco—was added. The original unaired pilot was also darker in tone, with Sheldon being more libidinous than how he's portrayed on the TV show now.

Thomas Dolby's "She Blinded Me with Science" was originally used as The Big Bang Theory's theme song, until it was replaced by one from Barenaked Ladies.

5. The Munsters

TV producers Allan Burns and Chris Hayward made a 14-minute pilot episode of The Munsters to pitch CBS on a new family sitcom. The network liked the pilot, but wanted to re-cast some of its actors before the series went to air. Yvonne De Carlo replaced Joan Marshall as the Munsters' matriarch (whose name is Phoebe in the original pilot, but changed to Lily when it was reworked). Network executives felt that her character looked too much like Morticia Addams from NBC's The Addams Family, the TV show CBS was trying to ape with The Munsters. Butch Patrick replaced actor Nate "Happy" Derman as Eddie Munster.

The Munsters' unaired TV pilot featured a different opening credit sequence and theme song, which was bouncier than the cool surf rock theme that stuck. And unlike the rest of the series, it was in color, which CBS ultimately felt was too garish and gaudy. The pilot never aired, but the footage was repurposed and ran as "My Fair Munster," the second episode of the first season. 

6. Buffy the Vampire Slayer

In 1996, 20th Century Fox and Joss Whedon produced a TV pilot for an adaptation of Buffy the Vampire Slayer, which Fox used to pitch networks on the new teen drama; the show ended up on The WB, bringing a lot of attention to the new network and quickly becoming its highest rated TV show. But before that happened, some changes had to be made: The logo was different; the name of the school changed from Berryman to Sunnydale; and the sets got a significant upgrade. But most notably, Riff Regan, who was originally cast to play Willow Rosenberg, was replaced by Alyson Hannigan. Stephen Tobolowsky played Principal Flutie in the pilot episode, too (he was replaced by Ken Lerner). The first official episode, "Welcome to the Hellmouth," aired on March 10, 1997. 

7. All in the Family

In 1968, TV writer and producer Norman Lear acquired the remake rights to the British sitcom Till Death Us Do Part with the hope of adapting it for American audiences. Originally, Lear titled the series Justice For All. It starred Carroll O'Connor and Jean Stapleton as Archie and Edith Justice (not Bunker), while Kelly Jean Peters and Tim McIntire played Gloria and Richard, her husband. ABC passed on Justice For All, but liked the premise enough to order a second pilot from Lear.

The following year, Lear re-tooled Justice For All with a new pilot called Those Were the Days. He kept O'Connor and Stapleton as Archie and Edith Justice, while Candice Azzara and Chip Oliver played Gloria and her husband, now named Dickie. ABC passed on the sitcom again and never aired either pilot.

Lear took the pilot for Those Were the Days to CBS, who had expressed interest in developing Till Death Us Do Part before Lear acquired the rights. The network loved it and immediately ordered 13 episodes. O'Connor and Stapleton stayed on as Archie and Edith (now) Bunker, while Sally Struthers and Rob Reiner were cast to play Gloria and Michael (a.k.a. Meathead) Stivic. Lear also changed its title again, to All in the Family, before CBS aired the first episode titled "Meet The Bunkers" on January 12, 1971.   

8. Family Guy

In 1998, Seth MacFarlane created a seven-minute pilot to pitch Fox on his new animated comedy. While the animation was crude, Fox loved the pilot and ordered Family Guy for the following year's fall TV schedule. A number of the pilot's jokes and story elements were recycled for the first official episode "Death Has a Shadow." Lacey Chabert voiced Meg Griffin throughout the show's first season, but was eventually replaced with Mila Kunis for contractual reasons.

9. Desperate Housewives

The original TV pilot of Desperate Housewives never aired because ABC was unhappy with the primetime soap opera's cast. Originally, Sheryl Lee played the role of Mary Alice Young, the TV show's deceased narrator. But producers decided to replace Lee with Brenda Strong. If Lee had remained on Desperate Housewives, it would've been the second time she played a dead character on a popular TV series on ABC, as she is perhaps best known for playing Laura Palmer on Twin Peaks.

Additionally, Michael Reilly Burke originally played the character of Rex Van De Camp, while Kyle Searles played John Rowland. Steven Culp and Jesse Metcalfe were re-cast in the roles, respectively.

10. Avatar: The Last Airbender

Avatar: The Last Airbender creators Michael Dante DiMartino and Bryan Konietzko made a short concept pilot as a way to pitch the animated series to Nickelodeon. DiMartino and Konietzko worked with Korean animation studio Tin House to make the pilot episode in 2003. While the animation was crude, the pilot included several elements that would become staples of the series, including its mythology, character design, music, and a few of its voice actors.

Dante Basco, Jack DeSena, and Mae Whitman appeared in the pilot, but her character's name was Kya instead Katara. Voice actor Mitchel Musso played Aang instead of Zach Tyler Eisen, who voiced the young Avatar in the animated series. Nickelodeon picked up Avatar: The Last Airbender for a three-season run in 2005.

11. 30 Rock

NBC wanted to re-tool the TV pilot for 30 Rock before ordering the comedy to series. Network executives didn't like Rachel Dratch in the role of Jenna DeCarlo, so 30 Rock's creator Tina Fey and producer Lorne Michaels re-cast the role with Jane Krakowski, whose name was changed to Jenna Maroney. Fey and Michaels re-shot the pilot and NBC ordered more episodes of 30 Rock for the 2006 fall TV season. Dratch appeared in the new pilot, as cat wrangler Greta Johanssen, and continued to portray different characters throughout 30 Rock's seven-season run.

12. Star Trek

Gene Roddenberry made a pilot for Star Trek titled "The Cage" in 1965, but NBC passed on it. Network executives believed that Star Trek's pilot episode was "too slow" and "too intellectual [with] not enough action." "The Cage" was produced by Desilu, whose owner, Lucille Ball, urged NBC to order another TV pilot because she believed in Roddenberry and Star Trek. A second pilot, called "Where No Man Has Gone Before," was made, and NBC ordered Star Trek to series.

While a new version and structure of "Where No Man Has Gone Before" aired in September 1966, "The Cage" and the original version of the second TV pilot remained unaired. "The Cage" also featured different characters and cast, most notably Captain Christopher Pike (Jeffrey Hunter) instead of Captain James T. Kirk (William Shatner). In fact, the only character that appeared in both pilot episodes was Mr. Spock (Leonard Nimoy). Actor Majel Barrett also appeared in the original pilot episode, but her role on Star Trek was downgraded from series regular to recurring. Footage from "The Cage" was re-purposed and used in the two-part episode "The Menagerie" during season one.