Addition by subtraction: Actors (or hosts) who improved a show by leaving

That was certainly the case when a pregnant Lisa Bonet left A Different World after season one. The departure of The Cosby Show’s Denise Huxtable from Hillman College allowed the show’s stacked cast more room to breathe, and gave the writers more room to mess around with characters like Jasmine Guy’s Whitley Gilbert and Kadeem Hardison’s Dwayne Wayne. It was only after Bonet’s departure that the show really took off creatively, with the strong ensemble cast taking on everything from date rape to the 1992 L.A. riots. While the ratings for the show dropped a little after the first season, A Different World became the most popular show in Black households starting in season two, a distinction that it held almost until the end of its run.

Parks And Recreation took some time to find its legs; newcomers to the series are advised to skip the six-episode first season and begin with season two, when the show’s creators wisely tweaked the characters, made the show less like their previous creation, The Office, and gave Parks a more upbeat tone. One aspect they kept in the mix was the love triangle with over-enthusiastic government employee Leslie (Amy Poehler), BFF Ann (Rashida Jones), and city planner Mark (Schneider). The triangle ran its course quickly, and at the same time Schneider had less and less to do plot-wise, his wry, laid-back humor was no longer a good fit for the more energetic program Parks had become. When Schneider left after the second season, it wound up being a boon to the show. In searching for his replacement, the show auditioned both Rob Lowe and Party Down’s Adam Scott, and ended up hiring them both. The new blood gave the show a shot in the arm, as the two quickly became an essential part of the ensemble—at least, until this week, when both Lowe and Jones leave the show. It remains to be seen whether their departure works out equally well.

When The Daily Show debuted in 1996, it was a fairly straightforward satire of news broadcasts, and the role of plastic, smarmy talking head played right to Craig Kilborn’s natural abilities. Kilborn continued on as host even after being called out for making horribly sexist comments about show creator Lizz Winstead, who quit the show. But despite the scandal, David Letterman tapped Kilborn to replace Tom Snyder as his lead-out. Losing a host can be a death knell for this type of show, but Daily traded up in a big way when Jon Stewart took over the show. As it adapted to the new host’s intelligence and political acumen, the satire became more pointed: It was aimed at newsmakers as much as news broadcasters. Stock interviews with B-list celebrities gave way to legitimately engrossing discourse with intellectuals and heads of state. Under Kilborn, The Daily Show was an amusing half-hour. Under Stewart, it’s become an institution.

One of the most famous departures in television history, Shelley Long quit Cheers at the peak of the show’s success to pursue a largely unsuccessful movie career. As half of the show’s central romance—one so important to the show’s success that “Sam and Diane” are to sitcom romances what “Kleenex” is to facial tissue—it was reasonable to think her departure mark the beginning of the end for Cheers. Look no further than Two And A Half Men or That ’70s Show to see how well sitcoms usually fare, creative and commercially, after a lead departs. Cheers’ producers saved the show with a series of smart moves. Rather than looking for a clone of Long’s prissy, intellectual Diane, the show found an entirely different foil for Ted Danson’s Sam in the form of overanxious striver Rebecca (Kirstie Alley). With a different type of character in the lead, and a different comic sensibility from Alley, the show deftly changed from a he-said/she-said screwball comedy to one of the great ensemble hangout shows in TV history. In retrospect, losing Long probably saved the show creatively, as the central Sam-and-Diane storyline was beginning to wear thin. Rather than having to milk more seasons’ worth of material out of the same central relationship, the writers were able to start fresh, in effect creating a whole new, equally successful show using the building blocks left over from Long’s departure.

It would be a grand understatement to say that ER’s original breakout star brought a certain amount of warmth and charm to Michael Crichton’s hospital drama. For five years, Clooney was the show’s main attraction, even as it became increasingly clear that his future held bigger things than being a TV heartthrob. His departure for the movies was supposed to be the beginning of the end for the show, especially as fellow cast members followed him to the exit in subsequent years. But instead of losing popularity, ER remained a hit, even with so many cast changes that by season 12, none of the original leads were on the show. Post-Clooney ER proved that the show’s format was bigger than any of its stars, as the series was second only to Law & Order as an enduring TV institution.

In its first season, the musical extravaganza Night Music (originally known as Sunday Night) featured an amazing number of fantastic guests, but struggled to find a coherent identity. Part of the problem was the decision to split the hosting duties between the leering pianist Jools Holland, with his “musical gangster” persona, and the easy-listening saxophonist David Sanborn. Under normal circumstances, it would be a fair assumption that Holland would be the host that was more fun to watch. But it soon became apparent that on a show where the guests were supposed to be the stars, and a typical episode might feature Sonny Rollins and Leonard Cohen dueting on “Who By Fire,” a little blandness at the center was exactly what was called for. By the start of the second season, Holland had packed his bags and returned to England, where he already had a successful career as a TV host, and Hal Willner had signed on as full-time “music coordinator,” which turned out to be the best trade since New York agreed to take that Babe Ruth guy off Boston’s hands.

When new-style cop show Miami Vice premiered in 1984, Gregory Sierra, who played the undercover-detective heroes’ boss, was the most recognizable face in the cast, thanks to his roles on such series as Barney Miller and Sanford And Son. His character—the stock warm, paternal precinct boss who sometimes yells at his bad boys for breaking the rules, but with the understanding that he’ll go to the mat for them—was highly familiar, too. Sierra held down the role for four episodes before deciding that the job was a non-starter and requesting that his character be killed off. His exit opened the door for Edward James Olmos, whose intense characterization of the replacement unit chief Castillo was the defining touch the series needed to distance itself from more conventional crime series. Olmos went on to win an Emmy and a Golden Globe for his performance.

George Dzundza originated the older-cop position on Law & Order as Detective Max Greevey, but grew increasingly frustrated when he realized that he was part of an ensemble show that would have little use for his character after the halfway mark of most episodes. He quit at the end of the first season, and was replaced by Paul Sorvino. Over the course of 31 episodes, Sorvino gave his character, Phil Cerreta, a degree of beefy warmth and intellectual cunning that Dzundza’s performance, which had seemed fine at the time, never came close to. But partway into the second season, Sorvino left to escape the rather hectic pace of a series shooting schedule. That cleared the way for Jerry Orbach to pick up the gauntlet as Detective Lennie Briscoe. Orbach gave a performance that made him the apotheosis of the senior-cop role and unofficial mascot of the NYPD. (Real cops loved the idea that this sharp-eyed cynic with a questionable past was one of their own.) On a show where heavy turnover among cast members had become the norm, Orbach played Briscoe for 12 years, right up until the actor’s death from prostate cancer.

The Vietnam War series China Beach was set at a seaside evacuation hospital and R&R facility, and featured an ensemble cast headed by Dana Delany as an Army nurse. The concept for the show was to depict the war as seen through the eyes of the American women who got closest to it. The character of the USO performer Laurette Barber (Chloe Webb, best known for her movie debut in Sid And Nancy) seems meant to provide viewers who had trouble identifying with career medical personnel or the prostitute played by Marg Helgenberger someone they could relate to. But the idea that women—or anyone—would see themselves in the USO singer is borderline insulting: She’s supposedly come to Vietnam partly because she thinks that doing tinselly covers of Motown hits for shell-shocked GIs will make her a star, and partly because a combat zone strikes her as a good place to meet guys. Webb, whose character did little but pull attention away from the real drama at the show’s center, left at the end of the first season and was replaced by Megan Gallagher, whose character—an ambitious war reporter—at least had a good reason for being there.

Marissa Cooper dies in a car accident at the end of The O.C.’s third season, but the character’s true cause of death was “evolution of show.” Mischa Barton’s poor little rich girl was a central figure in The O.C.’s early stages, when the show was positioned as a run-of-the-mill teen soap with agreeable taste in music. But as focus and fan enthusiasm shifted toward quirkier residents of Newport Beach—portrait-of-sarcastic-Barsuk Records-informed-angst Seth Cohen, Marissa’s deceptively cunning best friend Summer Roberts—Barton wound up swallowed by a run of hackneyed plots and increasingly “scandalous” love interests. In interviews following her departure from The O.C., Barton alluded to exhaustion with the show’s production schedule, though it’s just as likely that her character had exhausted her potential. In Barton’s absence, Autumn Reeser was bumped up to series-regular status; her insecure overachiever Taylor Townsend gave The O.C. greater opportunity to be the smartass meta-soap that was too often masked by Marissa’s weekly crises. It was a creative renaissance, but not a commercial one, and The O.C. followed Marissa into the dark after an abbreviated fourth season.

Archie Bunker may have outmatched Mary Richards in terms of ability to shepherd other characters into their own shows, but All In The Family never lost characters that were as crucial to its ensemble as Rhoda Morgenstern and Phyllis Lindstrom were to The Mary Tyler Moore Show. In a feat of tremendous TV fortitude, the most important female-driven comedy of the ’70s lost two of its bedrock actresses (Valerie Harper and Cloris Leachman) to spin-offs without stumbling. The key to Mary Tyler Moore’s continued success post-Rhoda and Phyllis were mid-series acquisitions Betty White and Georgia Engel. The energy and comic presence White and Engel brought to the show (White as vixenish TV host Sue Ann Nivens, Engel as sweet-but-naive Georgette Franklin) meshed with the brash personalities played by Harper and Leachman—but were also distinct enough to minimize the impact of Rhoda’s and Phyllis’ exits. White even managed to continue Mary Tyler Moore’s tradition of Emmy dominance, picking up the Outstanding Supporting Actress in a Comedy prize in 1975 and 1976—a category that was Harper and Leachman’s (and, in a one-time-only tie with All In The Family, Sally Struthers’) for the four prior ceremonies.

It would be unfair to blame Denise Crosby for the mediocre quality of Star Trek: The Next Generation’s first year, but her tenure as Tasha Yar is one of the many low points of the series’ debut season. Inconsistently written, Yar seemed grossly incompetent and undercut the show’s progressive vision of the future. The lack of attention paid to her character was Crosby’s reason for leaving the show, and after Tasha’s death, the producers shuffled around the cast and eventually retooled the series, giving Michael Dorn’s Worf and LeVar Burton’s Geordi larger, more interesting roles, both of which contributed some of Star Trek: TNG’s strongest ongoing plotlines.

While Grey’s Anatomy was initially built as an ensemble show centered on the hellish lives of a group of new doctors, as the show aged, it gradually took on more and more characters. Older actors came on as colleagues and love interests, and younger actors showed up as a newer, more naïve crop of interns. That’s why it wasn’t that big of a deal when Katherine Heigl got fed up with the behind-the-scenes drama and demanding schedule of the show and asked for the departure of her character, Izzie Stevens. Though Heigl seemed like one the more promising stars on Grey’s at the time, what with the success of films like Knocked Up and 27 Dresses, it turned out that the show was strong enough to withstand the loss of one of its key players—especially one whose character had taken such a turn for the annoying.