Revolving Door Casting

A show which suffers an excessive regular turnover in the main cast. Regulars might last for a season or two at the most and even the (seemingly) most vital characters have a very high chance of being dropped at the end of the year. This is not merely Loads and Loads of Characters - the core cast at any one time might be pretty small, just prone to being replaced.

Generally this happens most organically where a series is mostly concerned with a place of work such as a hospital or police station at which some turnover might be expected, but it is not limited to such. For obvious reasons this is much more likely to happen in a Long Runner.

The actual way the old characters are written out can range from merely Put on a Bus to Brother Chuck. Expect a lot of Suspiciously Similar Substitute style replacements. When a new regular conveniently shows up just as another is shown the door, that's Welcome to the Liberator.

Live-Action TV

 * Saved By The Bell: The New Class dropped three of its seven regulars at the end of its first year (including the main character). It did the same thing the following year, including a survivor of the first season. And so on, and so...
 * If you're interested:
 * Season 1 began with Mr. Belding, Megan, Weasel, Vicki, Scott, Lindsay and Tommy D.
 * For season 2, Weasel, Vicki and Scott were dropped and replaced with Screech, Brian, Bobby and Rachel. This was due to the six students in season one merely being expys of the original cast, and Saved By The Bell: The College Years getting cancelled lead to Dustin Diamond returning.
 * For season 3, Megan, Brian and Bobby left, and R.J., Ryan and Maria arrived.
 * For season 4, Tommy D, Lindsay, and R.J. were dropped and Eric, Nicky and Katie arrived.
 * For season 5, Rachel was replaced by Liz.
 * For season 6, Ryan was replaced by Tony. Seasons 6 and 7 were filmed concurrently so there were no further cast changes on the show. Only Mr. Belding was on the show from beginning to end.
 * Dream Team, the British soap opera set in a soccer club featured dozens of characters during its ten year run, the great majority lasting a year or so. And an absurdly high death toll for a soccer club.
 * Partially justifiable, in that much like in real life, players would sign for the club, before being sold on in a transfer window. Or, more than likely in the show, die.
 * Law and Order and its assorted spinoffs.
 * Seminal British police drama The Bill.
 * ER. Even Doctor Carter eventually left the show (as a regular).
 * To elaborate: during two or three seasons (with exception of some occasional episodes), NONE of the original series regulars were there. Curiously, all of them returned (briefly, mostly for only one or two episodes) for the last season.
 * Boston Legal revolves around this trope. No pun intended. The DVD interviews confirm this.
 * While the core group of interns in Grey's Anatomy has remained constant until recently, there has been a constant parade of attending surgeons and new interns. In the later seasons so many new characters appear and then rapidly disappear that the show appears to be just throwing characters at the audience to see who sticks.
 * Spooks has run for ten years and kills off or writes out main characters frequently and with extreme prejudice, something that starts in episode 2. Of the original cast only one remains, but he's more of the boss over the other main characters.
 * The process is slower than in other examples, but Degrassi has gradually retired its early characters in favor of new ones. Snake is the only one who's been a main character the whole time, and he's usually a minor character.
 * Damages tends to revolve around this trope with the appeal of the show generally being that most of the guest stars (and series lead Glenn Close) are film actors (William Hurt, Martin Short, John Goodman etc.) or other actors who normally don't do TV. In a given season these actors usually play the Big Bad or other characters that are somehow tied up in the plaintiff's case. At the end of the season a case is usually resolved and most of the supporting cast are either killed off or put on buses indefinitely. It's also notable that of the original  main characters so far only   have not been killed off or sent to prison with only a few characters added since then getting any luckier. The luck of the guest cast is almost worse.
 * Brazilian soap opera Malhação has had a lot of this since its inception in 1995 - even changing the setting (from a gym to a high school, then another high school) to justify it. The actor with the longest run (6 years) even joked he and the runner-up (5 years) stood so long because they never scored with anyone in the soap opera.

Anime

 * Due to the circumstances surrounding its creation, this occurred with Robotech, which changed casts whenever it began adapting a new anime and thus began a new cycle. Twenty-one years later, when Robotech: The Shadow Chronicles was produced, it replaced the cast again, combining a handful of characters taken from all three cycles with several new ones.

Comic Books

 * Any superhero team tends to fall under this.

Live-Action TV

 * Sabrina the Teenage Witch is a fairly mild case; only two main characters lasted to the end but most other characters stayed on the show for two to three years.
 * Power Rangers. Fairly justified in newer seasons that are actually completely different teams, but for the first several seasons, the main cast remained more-or-less consistent... except when it didn't, starting with the replacement of the red, yellow, and black rangers in season 2.
 * Let's review anyway:
 * Season 1 begins with Jason, Zack, Billy, Kimberly, and Trini. Tommy joins midway through.
 * Season 2: We have Rocky replace Jason as Red Ranger, Adam replace Zack as Black Ranger, and Aisha replace Trini as Yellow Ranger.
 * Season 3: Katherine replaces Kimberly as Pink Ranger, and Tanya replaces Aisha (although she never uses the Mighty Morphin' powers and doesn't become Yellow Ranger until Zeo.)
 * Season 4: There are only five rangers, so Billy steps down, leaving Tommy, Adam, Rocky, Tanya, and Katherine.
 * Season 5: Between seasons 4 and 5 (The Turbo Movie), Justin replaces Rocky as Blue Ranger. Midway into season 5, Tommy, Adam, Tanya, and Katherine are ALL replaced with TJ, Carlos, Ashley, and Cassie, respectively. At the end of the season, everyone heads into space on a rescue mission except Justin, who stays behind to be with his father.
 * Season 6: Andros appears and gives the team new powers, and then they begin replacing the ENTIRE cast each year.
 * Blakes Seven. Originally Blake, five other people and a computer. By the fourth series it was no Blake, five people, only two of whom were in the first series, and two different computers.
 * Soap opera Neighbours (and probably others of its ilk) is known for having a small cast of longer-running (usually older) characters and a substantial contingent of high-schoolers with a very high turnover rate.
 * Angel. At the end of the show, two of the original three characters, and at least one new character was accumulated every season.
 * Doctor Who. The new series replaced at least one primary cast member every single year.
 * In the 1960s, there were fairly frequent cast changes in calendar terms, though so many stories were done per year early on that short-term companions stayed through a significant number of episodes. Sara Kingdom and Katarina were both regulars for part of only one story, The Daleks' Master Plan.
 * Lost is notorious for its Anyone Can Die mentality that results in at least two main character deaths every season (except the first), plus several new regulars each year:
 * Season 1:  died and   was kidnapped/PutOnABus
 * Season 2: Ana Lucia, Eko and Libby join the cast.  all died, the latter two as part of a double murder.   was Put on a Bus.
 * Season 3: Ben, Juliet, Desmond, Nikki and Paulo join the cast.  both died..
 * Season 4: Daniel, Charlotte, and Miles join the cast.  returned from his bus trip and died.  wandered off into the jungle and vanished.
 * Season 5: No one joins the cast, though Ilana and Caesar were almost regulars; .  both died.  dies, but  stays on as.
 * Season 6: Recurring characters from previous seasons join the main cast,  dies, and  returns.  also appear.
 * Mission: Impossible (1966–73) was one of the earliest examples of this; only Barney (Greg Morris) and Willy (Peter Lupus) were regular characters for the entire show's run.
 * Earth: Final Conflict: The villain Sandoval (Von Flores) was the only character who stayed throughout the whole run.
 * Heroes was meant to switch out most of its main cast every season, but this was changed. Instead, everyone from season 1 has immunity from death or bus trips ( died but played a new character,  stuck around for half a season after  death). Even recurring characters that weren't promoted, like the Haitian, gets this immunity. Meanwhile, every volume introduces at least a dozen characters that appear in most, sometimes all episodes and are all replaced with a new set of characters. The only one that stuck was, ironically, introduced as one of a few one-off villains in the early half of his first volume.
 * Skins switches out its entire cast every two seasons.
 * Farscape: Following  departure in Season 3, at least one character was replaced with a new character each season.
 * 24. Only Keifer Sutherland has appeared in every episode, with the runner-up (Mary Lynn Rajskub) lagging behind by about fifty episodes. Some get jettisoned between seasons (Day 4 and Day 7 ditched most of the main cast except for Jack Bauer), but for the most part, characters departed the show by getting brutally killed off. Of the Main and Recurring characters from Day 1, only a tiny handful are still alive by the end of the series run, and only Jack Bauer is still actually in the show.

Web Original

 * Lonelygirl15. Danielbeast is the only character who was on the show from start to finish, and even he had some short phases of being Put on a Bus.